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EARLIER this year, I wrote about how to watch the faint planet Uranus disappear behind the moon, in a rare event known as a lunar occultation. Now, it is time for another lunar occultation, but this time of Mars (pictured, to the left of the moon).

Uranus, which is 2.6 billion kilometres away from Earth at the closest point in its orbit, is difficult to see with the naked eye. Mars, although much smaller, is far easier to spot. As our nearest planetary neighbour, Mars can get as close as 56 million kilometres away, at which point it is the third brightest planet in the night sky after Venus and Jupiter. So on 8 December, when Mars’s orbit will make it look as if it is disappearing behind the moon, it will be a spectacular sight – for those who can view it.

Occultations like this are only visible from certain points on Earth’s surface, since the moon is so close it appears at a slightly different position in the sky depending on where we view it from. This phenomenon, known as parallax, is easy to demonstrate. Hold your hand out with your thumb up and close one eye. Then open that eye and close the other and see how your thumb appears to move. This is because you are viewing it from a slightly different point – your other eye. Parallax is useful for measuring distances to nearby astronomical objects, but it also means the moon will only appear to be in front of Mars from some points on Earth.

Those of us who happen to live in a place where our view of the moon lines up with Mars, and where it is dark at the time Mars passes the moon’s orbit, will have a chance to see this month’s occultation. On this occasion, that happens to be in western parts of Europe, most of North America and some parts of north Africa.

This occultation is particularly rare because it falls on the same day Mars will be at opposition, meaning it is exactly on the other side of Earth from the sun. Planets at opposition appear brighter to us because their faces are fully lit by the sun, and they are visible for most of the night. Mars will next be at opposition in January 2025.

To watch this event, you will need to be prepared. Mars will appear to pass behind the moon and then reappear some time later. It will take about 36 seconds from when it looks as if Mars is touching the moon for it to totally vanish. It will be gone for about an hour, then take another few seconds to reappear fully. If you have binoculars or a telescope, these will give a clearer picture of the disappearing act, but it will also be visible with the naked eye.

The exact time the occultation will be visible for you depends on where you are. In the UK, it will begin just before 5am and end just after 6am. But to get a precise time, use software like Stellarium to plan your viewing, as even within the UK the timings will vary by a few minutes.

Abigail Beall is a features editor at ¿ìè¶ÌÊÓƵ and author of The Art of Urban Astronomy @abbybeall

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