Tips on Observing Mercury and Venus (The Inferior Planets)

In this article, I am going to examine the planets Mercury and Venus these planets are either always following the Sun at sunset or moving ahead of the Sun at sunrise, because of this these planets are always quite difficult to see.

Venus becomes visible just as the Sun sets while you can try and capture Venus and Mercury during the day though I do not recommend it as you can accidentally look at the Sun unless you know what you are doing.

 

Unfortunately neither Venus nor Mercury will show you much detail on their disks especially as Venus is covered in a thick cloud layer, however, I have managed to capture the clouds of Venus and the shades of Venus.

 

You will never see a full disk of Venus nor will you see a new Venus this is because when the full disk of Venus would be visible to Earth, Venus will be behind the Sun and when the new phase of Venus would be visible to the Earth, Venus will be in front of the Sun.

 

This happens for both inferior planets, Venus and Mercury and any other objects such as comets that happen to go inside the Earth's orbit so that point, they become inferior to us in the Solar System.

 

Even though the planet Venus is closer to Earth compared to the superior planets and therefore brighter. It is often more difficult to see compared to the superior planets due to the positioning of the planets.

 

There are a few concepts you need to get your head around if you want to see the inferior planets well first is the greatest Eastern Elongation this is the greatest height or altitude of the planet over the Earth.

 

Before the Sun rises in the eastern sky the Maximum Eastern Elongation is the best time to see Mercury or Venus in the morning sky. The next concept is the Superior Conjunction this is when the inferior planet is behind the Sun and so therefore impossible to see from the Earth.

 

The next is the Maximum Western Elongation and this is the opposite of the Maximum Eastern Elongation in that that is the maximum distance above the horizon in the Western Sky so when the Sun is setting, it is the best time to see the inferior planet at sunset.

 

The next concept to get your head around is the Inferior Conjunction this is when the inferior planet crosses the disk of the Sun and is therefore invisible from the Earth, however, this is rare as it requires the planets to line up just right so transits can be seen from Earth. 

This happened recently in 2019 and here is a link to the video these are quite rare events however a Mercurian transit is more common than the Venusian transit that happens about every hundred years or so as you can see in this footage of Mercury transiting the Sun.

 

Observing Tips for Mercury and Venus

 

First, never scan the sky unless the Sun is completely below the horizon there is a real danger that you could cause blindness. It is worthwhile to try and see Mercury, in fact, many astronomers never get to see it in their entire lives because it is always so low on the horizon and it is only there for such a short time.

If you live in a city, it can be difficult to get a clear horizon one thing to be aware of is that neither us nor Mercury will be colourful, targets will be bright but there is not that much surface detail.

 

When an inferior planet is at Maximum Elongation Mercury is between 18 and 28 degrees and Venus is between 45 and 47. However, as Venus and Mercury are low on the horizon they are always going to be affected by atmospheric dispersion. There are several ways you can get around this you can try an atmospheric dispersion corrector which, is a series of prisms that you align to counteract the movements of the atmosphere. Another way is to use a red filter which will block out blue light which is worst affected by atmospheric dispersion.

 

Another way is to use an IR pass filter if you doing astrophotography however if you are trying to capture the clouds of Venus a blue filter is quite useful because blue light brings out the details of the clouds of Venus.

To observe and Mercurian or Venusian transit you will need to convert a telescope into a solar telescope fortunately this is not too difficult.

 

If you would like to see what this looks like through the eyepiece as well as an animated version of how the inferior planets behave then please watch the episode below.

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