OCCULTATION

An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks from view (occults) an object in the background.


Lunar occultations of stars

While lunar occultations of planets are fairly rare events, the Moon occults bright stars quite regularly.

In fact, we can work out how many naked eye stars will be hidden by the Moon in a year by looking at the path the Moon takes through the sky.

This is centred on the apparent path of the Sun against the background stars, called the ecliptic, but tilted at a little over 5°.

The Moon covers around 0.5% of the night sky in its yearly journey, equating to about 850 naked eye stars including bright ones like Aldebaran, Regulus, Spica and Antares.

In any one month, you can be pretty sure that the Moon will occult a few decent stars.

When the Moon occults a star, it’s like the star has been switched off: one moment it’s there, the next it’s gone.

This is more noticeable before full Moon, when the Moon is ‘waxing’. Then, the part that first covers the star – the ‘left-hand’ edge – is dark and unlit.

It’s more impressive than seeing the star being hidden by the Moon’s bright edge.An occulted star will disappear almost immediately, and this shows that the Moon has no atmosphere.

If there were gases enveloping the Moon, an occulted star would shimmer and fade as it approached the lunar disc before finally fading out.