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BAA Tutorials Intermediate

Filters for visual observing of the Moon and planets

Introduction

If you are new to astronomy and wish to see as much detail as possible on the planets of the solar system, or even if one has been observing solar system objects for some time, it may come as a surprise to find that coloured filters can make a world of difference to your observing clarity. Using filters can revolutionize your observing as coloured filters bring out additional detail from the subtle shadings found on solar system objects. This tutorial builds on the excellent tutorial by Paul G. Abel, and looks in more depth at the filters most commonly used by visual observers of solar system objects.

Many astronomical suppliers provide these filters, and all filters are identified firstly by their colour, and secondly by particular numbers or a # which are known as Wratten numbers. These allow the observer to choose which parts of the spectrum they are going to enhance in order to make planetary and lunar definition stand out. The principle of the filters come from black and white photography in which complementary or “opposite” colours enhance the contrast visible. When juxtaposed, complementary colours make each colour seem more vivid and defined, enabling particular coloured features to stand out against the background hues. So, a red or orange filter will enhance blue features and a blue filter will enhance red features.

The Wratten system was developed in Britain in the early 20th century by Frederick Wratten and Kenneth Mees who founded a company in 1906 that produced gelatin solutions for photography. Mees then developed gelatin filters dyed with tartrazine to produce a yellow filter, but soon developed other colours and a panchromatic process of photography. In 1912 they sold the company to the American company Kodak, with their British offices at Harrow in England and Mees moved to New York to found the Eastman-Kodak laboratories there. In honour of his partner and mentor, Kenneth Mees named the burgeoning number of coloured filters “Wratten” and introduced the complex numbering system that is still in use today. Not all the Wratten filters are suitable for astronomical use, but the main colours are still widely used in visual astronomy and are detailed in this tutorial.

These coloured filters are known as broadband or “longpass” in that they allow a wide range of wavelengths through but block wavelengths above or below a certain point in the electromagnetic spectrum. As the spectrum of visible light lies between 390 and 700 nanometers (nm), with the blue wavelengths being the shortest (~400nm) and the red being the longest (~700nm). Anything with a wavelength range above or below a particular filter will be blocked and increased contrast in compensating colours will be noticed.

Most astronomical suppliers sell complete sets of filters for solar system observing and naturally such sets are known as lunar and planetary filters. They generally have a range from red to blue across the spectrum and cover the broad bandwidths associated with such colours. A typical set will include a neutral density filter for lunar observing and a No. 25 red, No.12 yellow and No. 80A blue for as full coverage as possible. A typical filter set is shown here in figure 1. A more extensive set of astronomical filters with typical Wratten numbers can be seen here in figure 2.

This tutorial will introduce each filter and instruct the reader on which targets in the solar system each filter can be used and what features the filters will enhance Keep in mind that visual acuity does vary from observer to observer and that in the dark the sensitivity of the human eye shifts to the blue end of the spectrum. This is due to a phenomenon known as the Purkinje effect, named after the Czech doctor who discovered that the spectral sensitivity of the human eye does not enable red light to be seen clearly in the dark, but shorter blue wavelengths are detected.

Technical aspects of Filters
Filters can be separated into a few main groups that enable enhancement, lessened contrast or can be used for colour shift or balance. Colour subtraction filters work by absorbing certain colours of light, letting the remaining colours through. They can be used to demonstrate the primary colours that make up an image or can be seen in the features of our planetary neighbours. A colour correction filter makes a scene appear more natural by simulating the mix of colour temperatures that occur naturally, and subtly enhancing the middle ranges of the spectrum.

In addition to these filters, there are also colour temperature filters. Some filters change the correlated colour temperature of a light source. They can change the appearance of light from a bright white source so that it looks more yellow and natural to the eye. The term colour temperature comes from the natural phenomenon of coloured light emitted by warm objects. Warm objects, such as a flame from a fire, emit deep red and orange light. The temperature of such flames are roughly 1500K. If you increase that temperature the light emitted begins to look more blue as its wavelength changes to the shorter (hotter) or blue end of the spectrum.

Of course, optical filters don’t really change the temperature of the object emitting the light. Colour temperature filters simply remove some of the light of wavelengths of our choosing so we can absorb or reflect away some of the orange and red light emitted by the planets. This makes the remaining light look more blue and therefore has a higher colour temperature. Conversely, some filters can remove some of the blue light emitted by a planet, making the remaining light look more orange and thus apparently emitting a lower colour temperature.

Wratten filters and their uses
In the following tutorial, I have grouped the filters under their colour designation rather than put them in number order, as the colour of each filter is their most obvious feature when using them. All of these filters are available to purchase in 37.1mm (1.25”) or 50mm (2”) fittings and are commonly available from astronomical suppliers. For a fuller description of Wratten filters, please follow this link: https://en.wikipedia.org/wiki/Wratten_number. In this tutorial, only those filters useful to astronomers will be described.

One question commonly asked is "do filters block out too much light and make observing more difficult or less enjoyable?" It is true that filters do block out some light, but I hope you will see from this tutorial that by selectively blocking out certain wavelengths of light, and by altering the contrast of any surface features, the observer is often able to resolve finer or more subtle detail. In fact, in the case of bright objects the reduction in light transmission is an advantage. Let us examine this a little more technically.

The difference in contrast between the belts and zones on an object such as Jupiter can be so small that the human eye and brain just smear the whole and it can be difficult to discern details without a filtered system. Because Jupiter is a very bright object seen against a dark background, the differences in intensity of reflected light from light/dark zones on such planets is not really seen to advantage by the human eye.

Contrast in any system can be measured using the formula:

C = (b2 - b1) ÷ b2

Where C is the contrast and b1 and b2 are different areas of brightness on the surface of a planet. Bright areas on Jupiter have an intensity of 6 lumens  m-2 and the intensity of the darker zones have an intensity of 3 lumens m-2. This would give:

(6 – 3) ÷ 6 = 0.5

or a visual contrast 50% lower in the darker zones than in the brighter zones. A filter will enhance the contrast by permitting wavelengths representative of the redder or darker zones through whilst diminishing the blue contrast on the brighter zones. Surely a filter that would aid in the perception of subtle features is going to be a bonus to any observer?

This tutorial will convincingly show that the use of filters, despite their decrease in light transmission is actually very useful in visual astronomy. The use of filters assists primarily in enhancing contrast initially and although the reduction in light transmission is generally not favoured in astronomy, this is one area in which this general rule need not apply.

No. 25 Red
The No. 25 filter reduces blue and green wavelengths, which when used on Jupiter or Saturn, result in well-defined contrast between some cloud formations and the lighter surface features of these gas giants. However, it needs to be used judiciously as the light transmission is only 15% but for such bright planets this filter will enhance the observed detail even when used with small telescopes. This filter blocks light shorter than 580nm wavelength. This filter is also sometimes referred to as a Wratten 25A.

No. 23A Light Red
This is a good filter for use on Mars, Jupiter, and Saturn, and has proved useful for daylight observations of Venus as it has a 25% light transmission. The light red is an “opposite” colour to blue and therefore darkens the sky very effectively in daylight. Some astronomers report that it also works well on Mercury, but I would not recommend viewing this planet in general during daylight due to its proximity to the Sun. This filter blocks wavelengths of light shorter than 550nm.

No. 21 Orange
This orange filter reduces the transmission of blue and green wavelengths and increases contrast between red, yellow and orange areas on planets such as Jupiter, Saturn and Mars. It brings out the glories of the Great Red Spot on Jupiter very well under conditions of good seeing with a medium magnification (e.g. x100). It also blocks some glare from the bright planet and provides less of a contrast between a planet and the black background of space. A good all round planetary filter as it transmits about 50% of the light and blocks wavelengths short of 530nm.

No. 8 Light Yellow
This filter can be used for enhancing details in red and orange features in the belts of Jupiter. It is also useful in increasing the contrast on the surface of Mars, and can under good sky conditions aid the visual resolution on Uranus and Neptune in telescopes of 250mm of aperture or larger. The No. 8 cuts down glare from the Moon and works much better than the “moon filters” included with some cheaper telescopes. This filter allows 80% of the light through but blocks light short of 465nm.

No. 12 Yellow
This filter works on the principle of opposites described above, blocking the light in the blue and green region and making red and orange features on Jupiter and Saturn stand out clearly. Deeper in colour than the No. 8 filter, it is the filter most astronomers recommend for visual work on the gas giants. It has a 70% light transmission and cancels some of the glare on Jupiter when seen against a dark background sky. It blocks visible wavelengths short of 500nm.

No. 15 Deep Yellow
This filter can be used to bring out Martian surface features, especially the polar caps and can be used to bring out detail in the red areas of Jupiter and Saturn. Some astronomers also have reported some success using this filter to see low-contrast detail on Venus. I have used this filter on Venus during the day to add more contrast to the image and it generally works well. This filter is particularly useful for visual observations of Venus as it is a very bright object and the filter can considerably reduce the glare of this very bright planet in evening or morning apparitions despite its 65% light transmission. The No 15 blocks light short of 500nm.

Although at this point it may feel like every filter suits Jupiter and Saturn, the variegated nature of their surfaces and their extreme brightness at opposition or during favourable apparitions enables a wide range of filters to bring out different details. Some of the details may be subtle, but can be explored better by an experienced observer equipped with a range of filters.

No. 11 Yellow-Green
This darker filter is a good choice to enable the observer to directly see surface details on Jupiter and Saturn. It can also be useful on Mars if you are using a large aperture telescope in the 250mm range. At times of steady atmospheric seeing, this filter darkens the surface features and makes areas such as Acidalia and Syrtis Major stand out and the polar caps and occasional features such as clouds appear quite marked. The No. 11 filter allows 75% light transmission can be used to darken some features on the Moon.

No. 56 Light Green
I have used this filter for observing the ice caps of Mars during its close encounter in 2003 and found that despite the low altitude of Mars from the UK during that apparition the filter worked well in bringing out these features and even hinted at rocky features on the planet’s surface during periods of clear seeing. I have to admit that the orange No 21 filter did work surprisingly well in rendering colour and detail on the red planet, but the contrast with the No 56 filter was quite good. This filter allows most wavelengths through but does have a peak around 500nm.

With its 50% light transmission this filter is a favourite of lunar observers as it increases the contrast while reducing the glare. It is also a filter that is well tuned to the wavelengths of the human eye and the greenish cast can almost be ignored during visual observation. This is a colour correction filter with all wavelengths equally affected. The effect can be seen on the first quarter moon in figure 3 photographed here in ordinary white light and then through the Wratten No 56 filter.

No. 58 Green
This filter blocks red and blue wavelengths of light and many observers find that it slightly increases contrast on the lighter parts of the surface of Jupiter. I have also used it on Venus where it does add to the contrast and reduces glare a little but it must be admitted that it is not easy to visualize any detail in the clouds.

The No 58 filter has a 25% light transmission, and it is a colour correction filter rather than a longpass. Such filters alter the colour temperature of the incoming light, enhancing contrasting colours in the object by allowing specific wavelengths through that correspond to the temperature of the light. This is a complex subject but to put it simply, the colour of light not only corresponds to particular wavelengths but also to particular colours where blue is cooler and red is hotter. Note that this is more of a perception than anything else as in reality blue light is “hotter” (has a higher frequency and shorter wavelength) than red light.

No. 82A Light Blue
This is almost a multipurpose filter as it does enhance some features on Jupiter, Mars and Saturn and also works very well in enhancing some features on the Moon (figure 4). It is commonly referred to as a “warming” filter that increases the colour temperature slightly and allows the red wavelengths through due to the complementary colours that we discussed above. With a light transmission of 75% it can be used on any aperture telescope and can even make some difference to deep sky objects such as M42 and M8 though the effects can be quite subtle. 

No. 80A Blue
Although this is quite a dark filter, it is as versatile as the No 82A in that it enhances features in the red on planets such as Jupiter, Saturn and Mars. It is also good for lunar observation as it reduces the glare and provides good contrast for some features such as ejecta blankets, ray systems and lava fronts. Some astronomers report success in its use on binary star systems with red components such as Antares and α Herculis as the contrast enables the observer to split the two components well. The No. 80A filter has a 30% light transmission and also acts as a colour conversion filter enhancing wavelengths around 500nm.

No. 38A Dark Blue
Again, a good filter to use on a planet such as Jupiter because it blocks red and orange wavelengths in such features as the belts and in the Great Red Spot. Some astronomers report that it also adds contrast to Martian surface phenomena, such as dust storms, and makes a better contrast for observing the rings of Saturn. Try using it for observations of Venus as some report that using this filter increases the contrast, leading to the visual observation of some dusky cloud features. This filter has about a 15% light transmission. It absorbs red, green and UV light and is commonly referred to as a minus green, plus blue filter. The difference can be gauged in figure 5.

No. 47 Violet
A very dark filter which strongly blocks the red, yellow, and green wavelengths. I would highly recommend it for Venus observation due to its low light transmission of about 5% providing great contrast and enhancing cloud features. Try using it on the Moon to decrease the glare when observing features at a 10-12 day old phase. Some observers report that features in the Schroeter Valley and Aristarchus crater are clearer due to the lack of glare. Recommended for the Moon, especially if you are using a large aperture telescope! This is another colour separation filter that enhances the blue or shorter wavelengths of the spectrum at 450nm.

Additional filters that also are helpful in visual observing are the polarizing filter and the neutral density filter. Both are longpass filters that usually transmit all wavelengths of light but can cut down on glare and contrast.

Non Wratten Filters
Neutral Density Filters
A neutral density (ND) filter transmits light uniformly across the entire visible spectrum and is an excellent filter to use to reduce glare in such objects as the Moon and planets, but especially the Moon. Due to its bright glare many lunar and planetary astronomers keep an ND filter on their favourite eyepiece and add on other filters as necessary. Neutral density filters come in a variety of densities that reduce the glare in the image based upon the amount of light transmission each ND filter allows. Commonly they come in numbers such as 50, 25 and 13 that signify the amount of light they transmit; 50%, 25% or 13%.

Polarizing filters
Although it does not work at any specific wavelength, the polarizing filter allows light of any wavelength through but blocks those with random scattering patterns allowing only light waves in a flat “plane” through, which has the effect of increasing the contrast, reducing glare and slightly enhancing the saturation of colour in an object. Such filters are very useful on bright objects such as the Moon and some planets.

Neodymium Filter
The Neodymium filter is an interesting addition to the filter armoury as it filters the yellow light of the spectrum, rendering most objects a faint blue colour. It is useful for observations of Venus and for Jupiter and Saturn too. Some astronomers report that this is a useful filter for observing in light polluted areas as it cuts through sodium light pollution somewhat, although it is not as effective as a Light Pollution Reduction filter.

Summary

Many planetary observers rely on filters and they report that they really do make a difference in seeing faint details. Filters also reduce the glare of objects like Jupiter, Saturn and the Moon and render a better contrast between their sunlit surface and the dark background sky.

Observers can also be affected by a phenomenon known as prismatic or atmospheric dispersion. This is most evident when a star or planet is seen near the horizon. It results from atmospheric refraction occurring less for the longer wavelength red light where the red appears clearer nearer the horizon and the light shifts to the violet toward the zenith. This is the reason that most astronomers prefer to observe an object when it is near or at culmination (the highest point in the sky as seen from an observers latitude) so that this effect is lessened. Use of red or blue filters on an ascending object may make the difference between seeing details such as the Great Red Spot for example.

I hope that this tutorial shows that coloured filters are a very useful tool in visual astronomy. Using such, I hope that this tutorial gives the reader some tips on which filters to use to observe any of the planets of the solar system and our moon. Most features on planetary surfaces may be quite subtle and filters can make a great difference between seeing or recording a feature or missing it completely in the sky background. For more information on using filters for visual observing or for astrophotography, please see my book Choosing and Using Astronomical Filters.

Martin Griffiths

Martin is the Director of the Brecon Beacons Observatory and an astronomer at Dark Sky Wales

[The graphical plots in this tutorial have been prepared using copyright free spectroscopic data]

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BAA Observing Sections Comet

Comet 21P approaches the North America Nebula

It will be a challenging observation due to midsummer skies and a First Quarter Moon but the periodic comet 21P/Giacobini-Zinner will be passing the bright emission nebula NGC7000 (the North America Nebula) in Cygnus between June 19 and June 21. The comet is currently around 13th magnitude but will brighten over the summer to become a potentially 6th magnitude object by September. At present the comet is a small fuzzy spot with a short tail to the south west and it will be completely overwhelmed by the large nebula but it will be interesting to compare the two objects. A chart showing the encounter is here. Please send any observations to the Comet Section.

The image at left shows the comet on the morning of 2018 June 13. The field of view is around 11 arcmin square. More images of this comet are available in the Section's archive here.

Update on 2018-06-18 - Here is an image of the comet approaching the nebula taken from New Mexico.

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BAA Tutorials Starting out

Noctilucent Clouds – a beginners guide

Noctilucent clouds (popularly referred to by the abbreviation “NLC”) are high atmosphere clouds which occur over summertime at mid latitude locations. They form at very high altitudes – around 82 km above sea level – and are, thus, a quite separate phenomena from normal weather or tropospheric cloud. They appear as thin streaks of “cloud”, often a pearly-blue colour, reminiscent of “mares-tail” cirrus cloud formations.

NLCs can be seen from around mid May to early August during the darkest part of a summer’s night when the Sun is between 6 – 16 degrees below the horizon. Typically, they will occupy the northern horizon, along the twilight arch, extending to an altitude of 10 – 15 degrees. Over the NLC “season” the bright star Capella dominates this part of the sky and serves as a good marker for the NLC observer. They used to be associated with northern UK but have been seen as far south as central France and they seem to be spreading further south with each season.

Observations of NLC remain of great value to professional scientists studying upper-atmosphere phenomena. Useful observations are very easy to make and require no special equipment.

The following information lists the important details you should include in your report:

LOCATION: Give the latitude and longitude of the place observations were made. Alternatively, give the name of the nearest town or city.
DATE: Use the “double-date” convention as used in reporting aurorae. That is, “June 21-22″ would refer to the night of the 21st and the early hours of the 22nd.
TIME: Try to use universal time (UT) even though British Summer Time (BST) will be in civil use for UK observers. Remember, UT = GMT = (BST – 1 hour).

Observations
The following features and details should be recorded at 15 minute intervals (i.e. on the hour, quarter past, half past and so on):

AZIMUTHS If you see NLC measure the left (western) and right hand (eastern) extent of the display. This is measured in degrees with west = 270, north = 000, east = 90 and south = 180. Polaris defines the northern point of your horizon. Azimuths can be gauged by using a clenched fist, held at arms length, as a measure of 10 degrees.
ELEVATION If possible, measure the angle subtended by the uppermost part of the display. A simple alidade can be made from a protractor and plumb line for this purpose.
BRIGHTNESS NLC brightness is measured on a three point scale with 1 = faint; 2 = moderate; 3 = very bright.

Structure
NLC forms are classified into 5 easily identified structures. Any combination of the following is possible:

Type 1: Veil – A simple structureless sheet, sometimes as background to other forms.
Type 2: Bands – Lines or streaks, parallel or crossing at small angles.
Type 3: Waves – Fine herring-bone structure like the sand ripples on a beach at low tide. Very characteristic of NLC.
Type 4: Whirls – Large-scale looped or twisted structures.
Type 5: Amorphous – Isolated patches of NLC with no definite structure.

Drawings
Simple sketches of the NLC can be very useful. These are best made in negative form with the darker parts of the sketch corresponding to the brighter NLC.

Photography
Photographs of NLC can easily be taken with a digital camera firmly fixed to a tripod; using 400 ISO gives good results. An exposure of 3-6 seconds with a lens aperture setting of f3.4 will normally suffice. However, it is always best to take several shots of different exposures, and pick the best exposure. Once this is done you can try a panorama by taking several overlapping photos. Make sure the camera is level, then move it about 20 degrees after each shot, starting just beyond one end. This makes sure that you will get it all, because the camera will see more than you can.

Sandra Brantingham

Sandra is the Director of the Aurora and Noctilucent Cloud Section of the BAA.

A selection of observatorions of noctilucent clouds which BAA members have uploaded to their Member pages can be found here.

[Thumbnail image by Gordon Mackie]

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Stargazers' Almanac 2019

Stargazers' Almanac 2019

A monthly guide to the stars and planets

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NGC 7331 and the "Deer Lick" Group

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About this observation
Observer
Kevin Gurney
Time of observation
19/09/2018 - 23:00
Object
NGC 7331
Observing location
Crique Campsite, Perpignon, France
Equipment
Celestron 8SE Scope
ATIK 460EX Mono Camera
Exposure
Baader Planetarium B 1" 1/4: 8x300" -10C bin 2x2 Baader Planetarium G 1" 1/4: 8x300" -10C bin 2x2 Baader Planetarium L 1" 1/4: 18x300" -10C bin 1x1 Baader Planetarium R 1 1/4": 8x300" -10C bin 2x2
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I have always found this galaxy and it's satellite galaxies to be a challenging object.  In this case Kevin has extracted lots of detail in the main galaxy, whilst picking out some of the structure in the smaller ones as well.  

Kevin's own notes give more detail about how the image was captured.

Location: Crique Campsite, Perpignon, France.  Date:Sept. 19, 2018 LRGB image - total time 3.5 hours.

Imaging telescope or lens:Celestron 8SE, Imaging camera:ATIK 460EX Mono, Mount:SkyWatcher AZ-EQ6GT, Guiding telescope or lens:Celestron 8SE, Guiding camera:Starlight Xpress Lodestar Autoguider, X2 Focal reducer: ASA 2" Reducer Corrector 0,77 Software:Hyperion Prism v10,  XnSoft XnView,  PixInsight 1.8 Ripley

Frames: Baader Planetarium B 1" 1/4: 8x300" -10C bin 2x2 Baader Planetarium G 1" 1/4: 8x300" -10C bin 2x2 Baader Planetarium L 1" 1/4: 18x300" -10C bin 1x1 Baader Planetarium R 1 1/4": 8x300" -10C bin 2x2

Pixel scale: 0.544 arcsec/pixel

Field radius: 0.218 degrees

Copyright of all images and other observations submitted to the BAA remains with the owner of the work. Reproduction of the work by third-parties is expressly forbidden without the consent of the copyright holder. For more information, please contact the webmaster.
BAA Tutorials Starting out

The size of things

Figure 1. The relative sizes of the Sun and planets. Source Wikimedia Commons, originator NASASpace is big, really big and in astronomy we often have to use some very large numbers indeed. So large in fact that they frequently become incomprehensible. In this tutorial we will try and bring the scale of the solar system, stars and galaxy ‘down to earth’.

What follows will be a couple of scale models in which we discuss the sizes and distances of various objects in the universe.

Figure 1 shows the relative sizes (not distances) of the Sun and planets of the Solar System. Notice in particular the wide disparity in planetary diameters and how the Sun (at the top) is so much larger than anything else.

Planetary orbits are generally not circular neither are the planets perfectly spherical. For example the distance of Mars from the Sun varies considerably as it orbits the sun and one look at the planet Jupiter will show it is much wider at the equator than it is from pole to pole. For the purposes of simplicity we will generally use mean distances and equatorial diameters but occasionally give the extremes as well in order to illustrate how non circular something is.

The figures used mostly come from NASA and unless otherwise stated, distances are measured from a body’s centre not its surface.

Earth, Moon and Sun
Let’s start in our own backyard so to speak. The planet Earth has a diameter of 12,756km. Although this in itself is quite a large number, the modern era of mass travel has made the size of our planet something most people are reasonably comfortable with.

For our purposes let’s shrink the earth down to a size of 5mm, about the size of a small pea. At this size the International Space Station orbits on average just 0.15mm above the surface of our mini-earth.

The scale size of the Moon is about 1.4mm in diameter or a little more than a quarter as large as the Earth. But how far away is it? Well the average distance from the centre of the Earth to the Moon is 384,000km so in our model the Moon is 151mm away. This is about 30 times the diameter of the Earth (Figure 2) or in terms of our scale roughly the length of a cheap ballpoint pen.
 Figure 2. This picture is an approximate scale diagram of the Earth-Moon system. Source Wikimedia Commons, created by Rizzoj from NASA data.
To put the Earth-Moon distance in perspective, let’s consider how long it would take to travel there. In 1969 the Apollo 11 spacecraft reached our satellite from Earth orbit in 3 days. A modern commercial passenger jet cruising at around 900 kph would take nearly 18 days to reach the Moon assuming it could travel in space and in a straight line. A vehicle travelling steadily at the UK limit of 70mph (113 kph) would take over 4.5 months to reach our satellite and someone walking non-stop would take about 9 years!

Moving further afield, the Earth orbits the Sun at distance of 149,600,000km. In our model this equates to a little over 58.6m. As for the Sun itself, it has a diameter of 1,391,016km. That is 109 times larger than the Earth and it would have a diameter of 545mm in our model, roughly the size of a large inflatable beach ball. The mean distance from the Earth to the Sun is known as the “Astronomical Unit” (AU) and will feature again in our scale model.

The table below summarizes the position so far.
ObjectEq. Diameter (km)Scale Diameter (mm)Diameter Relative to EarthRadius of Orbit (km)Scale Orbital Radius (mm)Orbital Radius Relative to Earth
Earth 12,756 5.0 1.0x 149,600,000 58,639 11,728x
Moon 3,475 1.4 0.27x 384,000 151 30x
Sun 1,391,016 545.2 109.1x - - -
In this table the orbital data for the Earth refers to its path around the Sun and for the Moon it is for our satellite’s orbit around the Earth.

The inner solar system
Figure 3. Relative sizes of the rocky worlds of the Solar System. Adapted from Wikimedia Commons.  Author: Koppelo, derived from NASA.Now we will reposition ourselves at the centre of the Sun and travel outwards through the Solar system. For simplicity we will only give the scale diameters in millimetres and distances in metres. The tables in this section will summarize the actual sizes and distances.

The innermost planets (the so called terrestrial planets) are all rocky worlds (Figure 3) and the first planet we encounter is little Mercury with a diameter of 1.9mm, not much larger than our moon and an average distance from the Sun of around 22.7m. However, the orbit of Mercury is distinctly non-circular and the distance varies from 18.0m to 27.4m.

Figure 4. The orbits of the inner planets out to Mars. Notice how the orbits of Mercury and Mars are distinctly non-circular with the Sun off-centre.After Mercury comes Venus with a diameter of 4.7mm not too dissimilar to that of the Earth although the conditions on Venus are wildly different to our home planet with surface temperatures of 470C and an atmospheric pressure 90 times that on the Earth’s surface (equivalent to being roughly 900m underwater in the Earth’s oceans). Venus orbits the Sun in a closely circular orbit with a scale radius of 42.4m.
 
Passing Venus we encounter the Earth 58.6m from the Sun before reaching the red planet Mars at a mean distance of 89.3m. However as mentioned earlier, the orbit of Mars is noticeably non-circular and varies from 80.9m to 97.7m. For comparison the Earth’s orbit is much more circular and its distance from the Sun varies by less than 2m in our scale model. Much smaller than the Earth, Mars comes in at just 2.7mm. Notice how in moving from the Earth to Mars we increased the size of the Solar System by over 50%. Figure 4 shows the relative sizes of the orbits of the inner planets.

After Mars we enter the asteroid belt. The largest of its denizens is the dwarf planet Ceres with a diameter of 0.4mm, much smaller than our moon and which orbits the Sun at a distance of 162.3m.

The table below summarizes the position so far.
ObjectEq. Diameter (km)Scale Diameter (mm)Size Relative to EarthRadius of Orbit (km)Radius of Orbit (AU)Scale Radius of Orbit (m)
Mercury 4,879 1.9 0.38x 57,900,000 0.39 22.7
Venus 12,104 4.7 0.95x 108,200,000 0.72 42.4
Earth 12,756 5.0 1.00x 149,600,000 1.00 58.6
Mars 6,792 2.7 0.53x 227,900,000 1.52 89.3
Ceres 950 0.4 0.07x 414,000,000 2.77 162.3

The outer solar system
Figure 5. The relative sizes of the four gas giants of the solar system. Adapted from Wikimedia Commons, originator NASA.We now leave the realm of the rocky planets and reach the region dominated by the gas giants (Figure 5) although nowadays Uranus and Neptune are often referred to as ice giants instead. First up is the solar system’s largest planet, the giant Jupiter. Jupiter is over 11 times the diameter of the Earth with a scale diameter of 56mm (roughly snooker ball size), orbiting 305m from the Sun.

Jupiter is the centre of a system of natural satellites. Of these, four are of significant size, Io, Europa, Ganymede and Callisto. These four satellites are known as the Galilean Satellites as they were discovered by Galileo and recognised as moons of Jupiter in 1610. They have scale diameters of 1.4mm, 1.2mm, 2.1mm and 1.9mm respectively. Notice that all except Europa are larger than our own Moon and that Ganymede is even larger than Mercury. Their details are in the table below along with those for Titan, Saturn’s largest satellite.
ObjectEq. Diameter (km)Scale Diameter (mm)Radius of Orbit around Planet (km)Scale Radius of Orbit (mm)
Io 3,643 1.4 422,000 165
Europa 3,122 1.2 671,000 263
Ganymede 5,262 2.1 1,070,000 419
Callisto 4,821 1.9 1,883,000 738
Titan 5,150 2.0 1,222,000 139
Beyond Jupiter is the lovely ringed world of Saturn at a distance of 562m from the Sun. The globe of Saturn itself will be 47mm across and the diameter of the rings to the outer edge of the A ring (the outermost ‘obvious’ ring’) is 107mm. For more on Saturn and its rings see this tutorial by Mike Foulkes. Like Jupiter, Saturn has a large retinue of moons. Of these the largest is Titan at 2.0mm. From this we can see that not only is Titan larger than our own moon but, like Ganymede, is larger than the planet Mercury as well.

Figure 6 The orbits of the planets to scale. The arrows on each planet show the direction of its axis. Source Wikimedia Commons, originator NASA.Moving on from Saturn we next come to Uranus, much smaller than either Jupiter or Saturn but still 4 times larger than the Earth at 20mm. Uranus too has several moons but all are relatively small. The planet orbits the Sun at a distance of well over 1km.

Next up is Neptune, slightly smaller than Uranus at 19.4mm but still a gas/ice giant in its own right orbiting at 1.76km.

Beyond Neptune we leave the realm of the giant planets and encounter what is generally known as the Kuiper belt but more properly the Kuiper-Edgeworth belt. This is a belt of cold icy worlds often in very elliptical orbits. The most famous of these is the dwarf planet Pluto. On our scale its diameter is 0.9mm (smaller than our Moon) and the orbit varies between 1.7km and 2.9km.
 
Figure 6 shows the relative sizes of the orbits of the outer planets with an inset for the inner planets.

The table below summarizes the position for the outer planets.
ObjectEq. Diameter (km)Scale Diameter (mm)Size Relative to EarthRadius of Orbit (km)Radius of Orbit (AU)Scale Radius of Orbit (m)
Jupiter 142,984 56.0 11.21x 778,600,000 5.20 305.2
Saturn - Globe 120,536 47.2 9.45x 1,433,500,000 9.58 561.9
Saturn - Rings 273,560 107.2 21.45x
Uranus 51,118 20.0 4.01x 2,872,500,000 19.20 1,125.9
Neptune 49,528 19.4 3.88x 4,495,100,000 30.05 1,762.0
Pluto 2,374 0.9 0.19x 5,906,400,000 39.48 2,315.2
At this point we will pause briefly and just consider how large the system of planets is in comparison to the size of our home planet. Recall that we started with the Earth represented by a 5mm sphere and have now travelled around 2.3km from the Sun to reach Pluto, a factor of about 460,000.

With this in mind we can now leave the Solar System behind and venture out into deep space.

Stars and galaxies
 Once we leave the Solar System, distances soon become very large indeed. The usual measure of distance is the light year, the distance light travels in one year. This is about 9,500,000,000,000km and using our scale of 5mm to the Earth’s diameter this would be about 3,725km. Bear in mind that as we travel out into the galaxy our measurement of sizes and distances becomes more difficult and the figures used represent best estimates but are subject to some uncertainty.

The nearest star to Earth after the Sun is Proxima Centauri at about 4.25 light years. This means that if we were to place our 5mm Earth in central London then Proxima would be 15,800km away, somewhere in SW Australia. Proxima is what is known as a Red Dwarf star making it both comparatively small and faint. It is nearly 202,000km in diameter, roughly 80mm on our scale or only about 50% larger than Jupiter.

Given that many stars are tens, hundreds or even thousands of light years away it is clear that with our original scale things will rapidly get out of hand. So let’s create a new scale that hopefully will give some feeling for the relationship between the sizes of the stars and the distances that separate them. This new scale will shrink the Sun down to a diameter of only 5mm. At this scale the Earth is now only 0.05mm in diameter.

With this new scale in hand let’s revisit Proxima Centauri. We now have two globes, one, the Sun, 5mm in diameter and the other, Proxima, about 0.7mm separated by over 145km. Truly, given their sizes, the stars are at immense distances from each other.

Red Dwarf stars are very common in the galaxy but because of their dimness they tend not to be very obvious. To get a feel for the sizes of the stars that we can easily see let’s look more closely at three of them that definitely are not Red Dwarfs.

Firstly, Sirius which appears as the brightest star in our sky. Sirius is 8.58 light years away which would be 293km on our new scale. It has a diameter of just less than 2.4 million kilometres which would scale down to 8.6mm, about 70% larger than our sun. Sirius is the seventh closest star system to Earth which helps explain its brightness.

Vega, the bright star visible in the summer skies of the northern hemisphere is 25 light years away (a scale distance of 854km) and over 2.8 times larger than the sun.

Lastly, another bright northern summer star, Deneb in the constellation of Cygnus the swan. Deneb is 2,600 light years distant (scale nearly 89,000km) and its diameter is about 203 times larger than our sun with an equivalent diameter of 1015mm.

So far we have just cherry picked some of the better known and brighter of the stars visible to us here on Earth. To get a better idea of the density of stars (or lack of it) consider that within a sphere of radius 10 light years (341km) there are just nine star systems. Of these, two are Brown Dwarf objects which may be thought of as “failed stars”. The table below gives details of these nearest systems.
Rank by distanceSystemDistance (Ly)Scale Distance (km)Notes
1 Alpha Centauri 4.37 149 Multiple star, the distance given is to the main pair. Proxima is an outlying member of the group at ‘only’ 4.25 light years.
2 Barnard’s Star 5.96 204 Red dwarf
3 Luhman 16 6.59 225 Brown dwarf pair
4 Wise 0855-0714 7.27 248 Brown dwarf
5 Wolf 359 7.78 266 Red dwarf
6 Lalande 21185 8.29 283 Red dwarf
7 Sirius 8.58 293 Brightest visible star from Earth. Double with a white dwarf companion.
8 Luyten 726-8 8.73 298 Double red dwarf.
9 Ross 154 9.68 331 Red Dwarf

Figure 7. An artist’s impression of the Milky Way galaxy with the Sun below centre. Source: NASA/JPL-Caltech/ESO/R. Hurt.Our Sun and all the stars we can see with the naked eye are only a small part of our own Milky Way Galaxy, (Figure 7) a huge assemblage of stars, gas and dust at least 100,000 light years in diameter. On our scale this would be roughly 3.4 million kilometres and our Sun is located around 26,000 light years from the centre or 888,000km on our 5mm scale.

Our Milky Way Galaxy is just one of billions and billions in the universe as a whole. If you go out on a clear night in autumn, you know where to look and the sky is dark enough you will see a small, faint patch of light in the constellation of Andromeda. This is M31, the great Andromeda galaxy. M31 is the nearest significant galaxy to our own at a distance of approximately 2.5 million light years. This would be 85.4 million kilometres on our scale.
 
Figure 8. The Local Group of galaxies. Source Wikimedia Commons, originator Antonio Ciccolella.Our galaxy and M31 are the two largest members of the ‘Local Group’ (Figure 8) a collection of 50+ nearby galaxies large and small. The diameter of the Local Group is around 10 million light years or over 341 million kilometres to scale.

This in turn is part of the Virgo Supercluster with a diameter of 110 million light years scaling to 3.75 billion kilometres.

So to summarise on our GALACTIC SCALE model:
  • The Sun is 5mm in diameter.
  • The nearest star is 145km away.
  • From the Sun to Deneb is 89,000km.
  • The Sun is 888,000km from the galactic centre.
  • Our galaxy is 3.4 million kilometres in diameter.
  • The nearest major galaxy is 85.4 million kilometres distant.
  • The Local Group is 341.5 million kilometres across.
  • The Virgo Supercluster is 3.75 billion kilometres across.
In conclusion
From all of the above one thing is abundantly clear, space is primarily just that – space. The sizes of the individual planets, stars and galaxies are dwarfed by the immensity of the distances between them. The 17th century French scientist and author Blaise Pascal wrote “The eternal silences of these infinite spaces fill me with dread”. With a little contemplation it is easy to understand how he felt although perhaps nowadays the overriding emotion is wonder rather than dread.

If you are in the York area at any time and would like a practical exploration of the scale of the Solar System, the University of York has created a scale model enabling you to cycle from the Sun to Pluto.

Lastly, in 1977, IBM produced a short but very famous film on the scale of things entitled “Powers of Ten”. It is perhaps a little dated now but is still well worth watching nonetheless to gain an appreciation of both the very large and the very small. It is available on the Internet and a search will turn up many copies. If you have not seen it I would encourage you to watch it.

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David Basey is an amateur astronomer living in semi-rural East Anglia. Primarily a visual observer he has been scanning the skies for over fifty years. On a scale of 5mm to the Earth’s diameter he would be rather less than one millionth of a millimetre tall.

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