Starwatch: Saturn’s yellowish hue will stand out during close encounter with moon

The moon glides to a close encounter with the ringed planet Saturn this week. The chart shows the view looking east-south-east from London at midnight as 24 July becomes 25 July, shortly after the pair have risen.

The moon will be a waning gibbous, 18.7 days old, and heading for its last quarter phase. It will have about 83% of its visible surface illuminated.

Saturn is experiencing retrograde motion, which means instead of steadily moving eastwards night after night as measured against the backdrop of stars, it is appearing to backtrack westwards. By November, it will have finished its loop and will again move eastwards.

Of course, Saturn is not doing this for real, it is an optical effect caused by the change of our perspective as Earth catches up with the outer planet and undertakes it. This year, the moment of undertaking, known as opposition, takes place on 8 September.

Because of its apparent backtracking, Saturn remains trapped in the constellation of Aquarius, the water bearer, until later this year. The planet’s distinctly yellowish hue will be easily visible during this month’s conjunction because of its contrast with the moon’s silvery light.

The conjunction is easily visible on the same night from the southern hemisphere as well.