Preceding, Following & Venus

[Lee Proctor]

Hi,

Apologies for what is undoubtedly a very pedantic post, but I’m really struggling to find the answer to this one.

Recently I’ve been getting more into planetary observation and have managed to totally confuse myself with regards to the terminology preceding and following.

My understanding of preceding/following with regards to the night sky is that as the sky turns from East to West, the westward direction is preceding and the eastward is following, straightforward enough.

With regards to the planets, when discussing Mars/Jupiter/Saturn, the East and West directions are reversed (so planetary East = celestial West and vice versa) so the preceding limb would be in the same direction as celestial west or planetary east (as the planets rotate from West to East).

Then we come to Venus, which rotates in the opposite direction.

1) Does this mean that planetary West on Venus is the same direction as Celestial West?

2) Which side is the preceding limb?

3) Does the term preceding relate to the rotation of the planet, so preceding can be either on the East or West side depending on which planet we refer to, or is preceding always associated with celestial west and the direction of rotation of the planet is irrelevant?

If anybody can clear this up for me that would be great as I feel like I’ve wasted too much brain power on this relatively insignificant detail already!

Many Thanks,

Lee

[John O’Neill]

Hi Lee,

My understanding of preceding/following is that it only refers to celestial west/east respectively. This is independent  of any rotation of the planet.

If you turn off the power on a telescope drive, a planet will drift in the Field of View in the preceding direction.

John

[David Basey]

Hi Lee,

John is correct in that usually preceding/following refers to the direction on the sky. It is particularly useful to record this on a planetary observation as it unambiguously establishes its orientation. It is convenient that the majority of planets rotate in a such a direction that celestial and planetary are broadly the same although the axial tilt may put them out of alignment. The extreme case is Uranus which ‘rolls around on its side’ and can even appear face on.

With regards to Venus I always report with celestial preceding not planetary.

There is one exception to all this. If you are using WinJUPOS software, for example to create an image measurement file, then that software uses planetary preceding and the Venus ‘P’ side is on the opposite side to other planets such as Mars and Saturn.

David.

[Lee Proctor]

Thank you both, this helps tremendously.

Looks like I will be sticking to preceding/following (celestial) for observations as it sounds like this will simplify the whole thing enormously.

Many Thanks,

Lee

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