See Uranus pass behind the Moon this month

By Pete Lawrence

Published: Monday, 05 September 2022 at 12:00 am

The Moon will appear to pass in front of the planet Uranus on 14 September, in an event known as a lunar occultation.

Lunar occultations of faint stars are common, but seeing the Moon move in front of a bright star is relatively infrequent.

Seeing the Moon pass in front of a planet is quite a rare event.

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Observing the lunar occultation of Uranus

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An illustration of the lunar occultation of Uranus, 14 September 2022. Times are correct for the centre of UK and will vary slightly depending on location. Credit: Pete Lawrence

On 14 September, the Moon will be at 77%-lit waning gibbous phase. Uranus, shining at mag. +5.7 will require binoculars or a telescope to see properly.

Locate the planet at 21:30 BST (20:30 UT) and familiarise yourself with where Uranus is positioned relative to the Moon’s disc.

At this time, the separation between Uranus and the eastern edge of the Moon (confusingly, the Moon’s western limb) will be around one apparent lunar diameter.

There’s nothing else of a similar brightness nearby, so identifying Uranus shouldn’t be too hard. Once you’ve located it, there’s nothing more to do than wait. 

The Moon’s bright limb slowly approaches Uranus until first contact, the time of which varies slightly with location.

It’s recommended to keep watching the planet when the Moon is close. From the centre of the UK, Uranus will make contact with the Moon’s eastern edge (western limb) at 22:30 BST (21:30 UT). 

What you’ll see

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It will take around eight seconds for Uranus to disappear behind the Moon’s leading, bright limb as occultation begins. Credit: Pete Lawrence

Uranus has an apparent diameter of 3.7 arcseconds and will take around 8 seconds to fully disappear.

Atmospheric seeing will have a big effect here, the tiny planetary disc being heavily influenced by Earth’s unstable atmosphere. 

Uranus remains hidden for around 50 minutes, the planet reappearing at 23:21 BST (22:21 UT) from behind the Moon’s dark western edge (eastern limb).

The period of time Uranus is hidden will also vary slightly with location, so observe the Moon’s dark edge earlier than the expected reappearance, say from 23:10 BST (22:10 UT).

Again, Uranus should take around 8 seconds to be fully revealed as the Moon moves east.

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After disappearing for 50 minutes, its return from behind the Moon’s following, dark limb will also take eight seconds. Credit: Pete Lawrence

What equipment to use

A telescope setup showing the entire Moon’s disc guarantees a view of the reappearance, although it’ll be hard to see Uranus as anything more than a dot.

More magnification will show the planet as a disc, but this increases the possibility of missing the reappearance.

If you have an accurate, polar-aligned mount, centring on Uranus at high magnification and sticking with it as the Moon performs the occultation is the best way to guarantee a high-powered view.

This guide originally appeared in the September 2022 issue of BBC Sky at Night Magazine.