The solar system - what's observable?

Hint: click the mouse on objects to see details & make unique finder charts.

2013 July: A non-Java version of this display is being developed, to avoid the slow and irritating start-up. A "beta" test version can be seen here.

The main aim of this page is to give observers an easy-to-read summary of where solar system objects are now and will be in the near future, and whether the Moon is likely to interfere with observing them. Thus it is a tool for the initial stages of planning observing sessions. An observing list can be compiled (with RA, Dec, phase angle, etc) in a format that can easily be pasted into a spreadsheet.

The plot shows the whole of the celestial equator and a band 45 degrees north and the same south of it. It is plotted in such a way that the Sun is always at both ends. The vertical scale divisions are 10 degrees of declination. The top scale shows right ascension (RA) and the bottom scale shows elongation in degrees eastwards from the Sun. Elongation 180° is the meridian at midnight. Any planet on the meridian is at opposition. Objects to the right of it are evening objects, those to the left are for observing in the morning.

Initial letters are used for identifying the planets. The two M's are distinguished by Mercury having a small letter and its dot being cyan, whereas Mars is orange.

The short green lines through the Sun are tangents to the horizon at sunrise and sunset for the given latitude. The parallel grey lines indicate the astronomical twilight zone: objects between the grey and green lines will only be seen in twilight (the Sun is not yet 18 degrees below the horizon).

The applet attempts to get your latitude from your system by finding your country and looking up its mean latitude. However, if that does not work, or it is not accurate enough, you can click the button and enter a different value. The latitude value only affects the angles of the horizon and twilight markers. In the dialogue for setting latitude you may also choose whether to have a northern hemisphere view (N up, E left) or a southern one (S up, E right).

The sizes of the Sun and the Moon are exaggerated, so it means nothing if planets appear in front of them (Sun and Moon are always plotted before planets).

Data summary for any object

Moving the mouse cursor over the plot highlights the nearest object and displays its name in full. Clicking the mouse when a name of interest is shown causes a window to pop up containing summary data for the object including its position (RA and Dec), rising and setting times, etc. Data for the Sun also include twilight start and end times.

The data summary window also includes some option buttons, depending on the object type. In most cases a much more detailed chart showing just a few degrees around the object and stars down to about magnitude 11 can be plotted. The text labels on the detailed chart may be moved with the mouse if they are obscuring something useful.

Data in the summary window are designed to be copied easily. Click the cursor in the text box. In Windows then use Ctrl-A to select all the text and Ctrl-C to copy it to the clipboard. From there it can be pasted into anything that accepts text. In other systems there will be a similar process but with different key combinations. In this way you can compile a list of objects that are favourably placed for observation and go on to research their details, perhaps via Google.

In fact the applet itself maintains an observing list. Buttons in the data display window make it easy to add an object's data as a new row in the list and to view the whole list. The observing list is in CSV (comma-separated values) format. Spreadsheets accept CSV data to make a proper table. To copy the list to a spreadsheet use Ctrl-A (Windows) to select it all, then Ctrl-C to copy it to your system's clipboard. From there it can be pasted into a spreadsheet or any text editor.

Adding objects to the display

You can add further objects to the plot by means of the "Add object" button and remove them all again with the "Clear" button. "Add object" presents several choices:

The lists contain hundreds of minor planets and comets and the majority of Messier objects that lie in the equatorial band. It can be useful to add a whole list first. That enables you to see which objects are in a suitable part of the sky for your latitude, so they are likely to be observable. You can then clear the list and concentrate on the most interesting objects. Type the initial letter of a name to get into the right part of a long list. The list of minor planets comprises all those reaching magnitude 12.0 or brighter at the next opposition.

The Kreutz search area refers to the position in the sky which is likely to be the best place to look for comets of the Kreutz group, ahead of opposition, giving you a chance to be the first to discover one! The theory of this is given in two papers by Brian Marsden:

There is more introductory information about the Kreutz Sungrazers in the book

Our applet gives considerable information about the likely positions of Kreutz comets at various numbers of days ahead of opposition. The details may be charted and tabulated.

If you choose to add "Any sun orbiter", a dialogue opens for entering a set of heliocentric orbital elements (elliptical or hyperbolic but not parabolic or circular: you cannot use eccentricities of exactly 0 or 1). Published orbital elements vary in the selection of values provided, so the dialogue allows different combinations. Eg, depending which of a, q or n is known, just enter that value. Similarly, you may know T, L, or M for defining an object's position at a known time but the dialogue accepts any one of those. For L or M it is essential to enter the epoch too. Times are entered as Julian Dates, so a JD/calendar converter is also provided.

For perihelion, check carefully whether the value you have is for the longitude of perihelion or the argument of perihelion. They are not the same thing. Unfortunately some published data do not specify which of those two values has been given. (In fact our own dear Handbook is guilty: the tables of orbital elements for asteroids and dwarf planets both have a column headed "Peri". The values beneath that are in fact arguments of perihelion. Also, to be strictly accurate, the column headed "Node" is for the longitude of the ascending node.)

The summary data window for a minor planet or comet includes the values of r (distance from the Sun), Δ (delta, distance from the Earth) and the phase angle between the two vectors to Earth and Sun. For comets 5.log10(r.Δ) is shown, which is relevant for magnitude estimation. The display for a minor planet includes a V magnitude estimate based on H, G, r, Δ and phase angle. The theory behind this was explained in "The H and G magnitude system for asteroids" by Roger Dymock in the BAA Journal: J.Br.Astron.Assoc. 117,6,2007 p.342. The applet uses the formulae given in that paper.

The last two kinds of object that can be plotted are static (non-orbiting) objects. The aim is to help beginners in particular to see when any object is likely to be observable. Once the name of a static object has been entered the applet automatically asks the Simbad service at CDS Strasbourg for the coordinates of the object. In naming such objects 3-letter abbreviations may be used. For example, to plot Alpha Orionis it is sufficient to put "alp ori". The full naming rules can be seen on the Simbad web site.

The Project button in the object addition menu asks for a password. It is for special projects set up by BAA Sections. Ask your Section Director whether there is a relevant project.

Algorithms used

The Earth's heliocentric position is calculated according to the VSOP87 theory, as described in Chapter 32 of Astronomical Algorithms by Jean Meeus (2nd Edition). This method is not used for plotting the other major planets on the grounds that it may take too long to compute on the average PC. However, if you click to see the details of a major planet its position is then recalculated accurately for the numerical display. The first time a major planet is clicked there may be a slight delay as the VSOP87 code and data are downloaded. (This has been modified 2011 March so that the accurate calculation is always done for all the major planets if the date is outside the years 2000 to 2014. So current plots are fast but accuracy is kept for thousands of years into the past or future.)

The position and phase of the Moon are calculated using Chapront's theory as described in Chapters 47 and 48 of Astronomical Algorithms by Jean Meeus (2nd Edition). So the applet displays accurate positions for the Sun, Moon and major planets (accuracy about 1 arc second).

The calculation scheme for minor planets and comets is instead based on theory given in "Orbital Motion" (3rd edn) by A.E.Roy, Institute of Physics Publishing, UK, 2002, ISBN 0-85274-229-0. The calculation uses the latest osculating orbital elements from the Minor Planet Centre. This is accurate enough for this small image display and for a few years around the present time. It is not useful for dates many decades or centuries from the present.

Applet written by Graham Relf - see