The phases of Venus

Venus, the second rock planet from the Sun, is commonly known as the morning or the evening star. It was the ancient Mesopotamians who were the first to realise that both ‘stars’ were, in fact, the same object.

It has long been known that Venus is covered with a thick blanket of clouds which never clear away, continually hiding the surface from view. Spacecraft have revealed that the extensive atmosphere is composed of carbon dioxide, with a sulphuric acid rain falling between the cloud layers. The clouds trap the Sun’s heat, and this ‘runaway greenhouse effect’ has transformed Venus into a raging inferno with a surface temperature of about 462°C.

Although we can’t observe the surface from Earth, the Pioneer Venus and Magellan spacecraft revealed a striking surface which is clearly the product of volcanic activity – indeed it is estimated that there are seven times more volcanoes on Venus than on the Earth (although it is open to debate if any of them are still active). Many of them appear rather like pancakes, and the relatively flat surface seems covered with a number of striking geological features. The planet has presented us with many mysteries, which are likely to keep planetary scientists occupied for many generations!

Venus has acquired a reputation as a difficult object to observe – when viewed in a dark sky the planet is so bright that the glare washes out any delicate cloud features, revealing little more than a blank white disk. The best time to observe Venus is at dusk in a bright sky, when the brightness of the background sky significantly reduces the brilliance of the disk.

Although the cloud features of Venus are somewhat vague, they can be made out with practice (particularly if your eyes are more sensitive to the bluer end of the spectrum). It is not uncommon for either one or both of the poles to be covered with a bright polar hood – these tend to come and go and vary in size as well as brightness. Filters are a great help when observing Venus: most telescopes come with a selection of optical filters, and they work by allowing some wavelengths of light to pass through while stopping others. Filters are numbered according to the Wratten scale and you will find the number printed on the side (a W25A is red for example, a W15 is yellow and so on).

The BAA’s Mercury & Venus Section has been collecting and analysing amateur observations of Venus since the foundation of the Association in 1890. If you go to the Section website, you can find the full observing programme, together with observing forms to record your observations.

The phases of Venus

Figure 1. The orbit of Venus as it moves around the Sun as viewed from earth
In 1610 September, Italian astronomer Galileo Galilei turned his small telescope towards the planet Venus, and discovered that the planet had phases just like those of the Moon. This was further evidence that Copernicus’ heliocentric (sun-centred) model of the solar system was correct. Galileo followed the planet and observed the half phase as well as gibbous and crescent phases, which would be impossible to observe if the old Ptolemaic Earth-centred model was correct.

Both Mercury and Venus exhibit a phase because they are closer to the Sun than the Earth: astronomers call them inferior planets (not as a derogatory term, it simply means their orbits are inside that of the Earth.) In contrast, planets with orbits larger than the Earth’s (Mars, Jupiter, Saturn etc.) are called superior planets.

This distinction between planetary orbits may sound rather abstract, but it does make an enormous difference to how the planets behave in the night sky. For example, periodically a superior planet comes to ‘opposition’, i.e. the Sun, Earth and planet all lie (roughly) on a straight line with the Earth in the middle. In contrast in the similar situation an inferior planet is either behind the Sun or between the Earth and the Sun. A superior planet can appear in the sky all night, whereas an inferior planet can only be seen in the evening sky (setting before midnight), or in the early morning sky (rising after midnight).

Astronomers speak of an apparition, usually taken to mean the period of time for which a superior planet is visible in the night sky. For inferior planets we have elongations instead – Mercury and Venus are visible either in the evening sky, in which case they are on the eastern side of the Sun and so said to be in Eastern Elongation, or alternatively, if they appear in the morning sky on the western side of the Sun they are said to be in Western Elongation.

As Venus orbits the Sun, the phase of the planet appears to change (rather like those of the Moon). The phases of Venus and the idea of elongations are easier to understand if we use a picture. In Figure 1 an illustration showing the orbit of Venus as it moves around the Sun.

Starting at position 1, Venus is in superior conjunction with the Sun and has a phase of 100%. From Earth, the planet is very close to the Sun in the sky and is effectively not visible. As we move to position 2, Venus starts an eastern elongation and now appears in the evening sky – a 60mm telescope is sufficient to reveal that the planet has a waning gibbous phase. The phase continues to decrease and we reach position 3 which is dichotomy, or 50% phase.

Venus continues to move closer to us, now appearing as a waning crescent, and notably larger in size. During this part of its orbit, Venus will reach greatest eastern elongation – the greatest separation between Venus and the Sun in the evening sky. Eventually we reach position 5 where the planet is between the Sun and the Earth – this is called inferior conjunction and the planet is once more too close to the Sun to observe. In theory the phase of Venus is now zero, but very often the sunlight is scattered by Venus’ atmosphere creating a ring of light which may or may not be completed. These are called cusp extensions.

After superior conjunction Venus reappears in the morning sky, now embarking on a western elongation. Telescopically, Venus appears as a waxing crescent, the phase steadily increasing until greatest western elongation is reached (the greatest separation between the Sun and Venus in the morning sky.) At position 7 Venus has reached dichotomy, and the phase continues to increase, now waxing gibbous. The planet now appears much smaller and is getting harder to observe until it eventually reaches inferior conjunction and the whole cycle then starts once more.

The phase anomaly

It is always the case that the observed phase of Venus is slightly less than the theoretical one. For example, dichotomy (or 50% illumination) is  predicted to occur on a specific date, If you observe Venus on this date, you will see that the phase is more like 48% and observed dichotomy will take place a few days later. The effect is much more pronounced in a blue filter (like a W47), and smaller if the planet is viewed with a yellow one (W15). This effect is called the phase anomaly of Venus and was first recorded by the German astronomer Johann Hieronymus Schröter in 1793.

The phase anomaly is caused by the thick Venusian atmosphere – the clouds scatter the sunlight, and blue light is more affected than red. As a result, the phase anomaly is very pronounced when Venus is viewed in blue light or imaged in ultraviolet. The Venusian atmosphere is rather dynamic – it rotates about the entire surface in just under four days, so the extent to which phase anomaly is observed varies somewhat, although it is always present.

Measuring the phase

To measure the phase, simply measure the distance from the limb to the terminator and divide by the diameter of the blank. Multiply your result by 100 to give the phase as a percentage
Measuring the phase of Venus is an easy task and requires nothing more advanced than a ruler and a drawing of the planet. First, print out an observing blank (available on the Mercury & Venus Section website), and take it out with you to the telescope. Spend some time examining the planet in your telescope, noting what features are present and so on. You will find your eyes will gradually become better at picking out the subtle details in the Venusian atmosphere.

When you’re ready to make a disk drawing, simply draw on your blank the terminator which you think accurately represents the phase that you can see. You should also record any cloud markings if you can see them – I find a 3B pencil suitable for this. When you have finished, make sure you record the date and time and any other details which the form requires.

You can measure the phase indoors after your observing session. Use a ruler to measure the distance from the limb, through the centre of the disk, and out to the terminator (see Figure 2). The Mercury & Venus Section blank has an overall diameter of 50mm, however you need to make sure your printer hasn’t rescaled the blank when printing (you can check by measuring the distance from the north to south poles).

In Figure 2 the distance measure is 32mm, so we have:

Phase = 32/50 x 100 = 64

and hence the measured phase is 64%. If your blank had an overall diameter of 60mm instead then we would have:

Phase = 32/60 x 100 = 53

i.e. a phase of 53%.

It is a good idea to estimate the phase every time you observe Venus, ideally over the whole of the elongation if you can manage it. If you’re going to do this as part of the Mercury & Venus Section programme, you will need to make the drawing using a yellow filter so as to reduce the effect of the phase anomaly. Personally, It is recomended that observations of the phase in white light, yellow, blue and red. You will also find that any cloud markings present will look subtly different in different filters as we are looking into different depths of the atmosphere.

Paul G Abel

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