BBC Sky at Night Magazine

Moon occults southern part of Pleiades

BEST TIME TO SEE: 19 October from 19:30–22:45 BST (18:30–21:45 UT)

The Moon’s passage across the night sky periodical­ly moves it closer to certain stars and objects. To experience a lunar encounter, such objects must be within a region of sky approximat­ely 10° wide, centred on the ecliptic. Over recent months, for example, the Moon has been making relatively close passes of the beautiful Pleiades open cluster, M45, in Taurus. Occultatio­ns of the cluster will eventually occur, the visibility of which will be down to whether they happen at night with the Moon above the horizon.

A taster occurs on the evening of 19 October, when a 91%-lit waning gibbous Moon passes immediatel­y south of the main part of the cluster. To the naked eye, the proximity of such a bright Moon to the much dimmer cluster stars will render the alignment unseen. Binoculars should fare better, with the brighter cluster stars easily visible to the north of the Moon. A telescope will do better still, bringing out many of the cluster stars near the Moon.

Using a telescope, it should be possible to see the Moon glide in front of some of the cluster’s southern outliers, offering an excellent opportunit­y to practise some occultatio­n timing observatio­ns. These are relatively easy to do, but can catch you out if you’ve never seen one before. Since they’re points of light, stars disappear in an instant behind the airless edge of the Moon – so fast that it’s common to be watching a star intently and blink for an instant, only to find the star disappeare­d during your blink!

If it’s clear on the evening of 19 October and you fancy having a go at making some timing observatio­ns, you can find more informatio­n about how to do this on page 55.