How to calculate and observe lunar occultations

Make records of stars and planets as they disappear behind the Moon.

lunar occultation of mars

In astronomy, occultations are when one Solar System body moves in front of a more distant astronomical object.

Most commonly occultations involve stars or a planet, but they can be, for example, an asteroid occulting a star.

The common factor is that light from the distant object is blocked for a short time as the occulting body passes in front of it.

As the Moon orbits Earth, it hides stars and planets, and these are known as lunar occultations.

The Moon moves within 5° either side of the ecliptic and so doesn’t occult the same objects all the time.

Mars occultation trails, Tomáš Slovinský, Kosice, Slovakia, 8 December 2022

Occultations are time- and place-dependent, so not everyone will be able to see the same occultation, and those who can will see it at slightly different times.

Lunar occultations have given observers the chance to discover double and multiple stars.

First-magnitude Antares, for example, hides its fifth-magnitude companion Antares B in its glare, but during a lunar occultation when the main star is hidden behind the Moon, the companion star becomes briefly visible. 

During an occultation the star’s light will ‘go out’ instantly as if someone had just switched it off.

If the star being occulted happens to be close to the north or south poles of the Moon, then a ‘grazing occultation’ occurs, where the star disappears and reappears behind lunar features seen in profile on the Moon’s edge or limb.

Lunar occultation of Venus. Credit: Pete Lawrence

Observing a lunar occultation

Occultations are best observed when they take place at the dark, unlit, limb of the Moon.

Disappearance is easier to observe, as you can watch the Moon and star get ever closer.

To document your observation and send the information to an astronomical society or scientific body, record your location, equipment, the occulted star and time of the event, as well as anything unexpected you observed. 

Along with a telescope, to record an event you can use a stopwatch and an accurate clock with a seconds readout.

You won’t be able to watch the occultation and keep an eye on what the real time is, so keep your eye on the sky. As soon as the occultation happens, start the stopwatch.

Later, at a suitable moment, look at your clock and stop the stopwatch, noting that time down.

Deducting the stopwatch time from the clock’s time will tell you the exact time that the occultation began. 

Occultations are easy and satisfying to observe, especially for novice observers, and it is something that can be done on those nights when the Moon’s light takes deep-sky targets off the menu.

Uranus occultation, by Agapios Elia and Siegfried Trattnig, Cyprus, 14 September 2022

Equipment for predicting and observing occultations

Software for predicting lunar occultations, available from the International Occultation Timing Association (IOTA), occultations.org.

Publications that give lunar occultation predictions of brighter objects but not location-specific timings. For example, BBC Sky at Night Magazine or those from national astronomy societies 

A stopwatch to record the time of an event

An accurate clock with a seconds readout

A telescope; a useful size is a 4-inch refractor or larger, depending on the brightness of the object being occulted

A template for recording your observations or an astronomy observation notebook

Observing a lunar occultation, step-by-step

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Step 1

how observe lunar occulations step 01

Download lunar occultation prediction software such as Occult v4.

Input your location details and a date range to get a list of occultations specific to your location.

Astronomy publications and local and national astronomical societies can also help.

Step 2

how observe lunar occulations step 02

Choose which lunar occultations to observe.

Determine where on the lunar limb the star will make its disappearance and reappearance. 

Disappearances on the dark limb are easier to gauge, as you watch the star as the Moon moves ever closer.

Step 3

how observe lunar occulations step 03

Check the location you are observing from and make sure you can see the Moon at the time of the event.

Set up your equipment.

Make sure you are familiar with moving it steadily (if it is not motorised).

Practice using your stopwatch.

Step 4

how observe lunar occulations step 04

Observe the occultation.

Make sure you watch the star as it moves towards the Moon during a disappearance, or that you are observing the area of the lunar limb where the star should make its reappearance, as the limb itself won’t be visible if it’s in shadow.

Step 5

how observe lunar occulations step 05

Start your stopwatch as soon as you see the star disappear.

Then, looking at an accurate clock, stop the stopwatch, noting the precise time.

Deduct the stopwatch time to pinpoint the start of the event at your location.

This also works for the reappearance.

Step 6

how observe lunar occulations step 06

Record the details of the event either for yourself or to send to the relevant bodies, such as IOTA, if required.

As the software predicts an event time, you may find that there is a difference to the time that you observed the occultation.

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Have you observed or even photographed a lunar occultation? Let us know! Email us via contactus@skyatnightmagazine.com

This guide originally appeared in the January 2023 issue of BBC Sky at Night Magazine.