The big three
BEST TIME TO SEE: 6, 14, 18 & 25 September
There are many different ways to define a coming together of two or more heavenly bodies in the sky. The general term for this is a conjunction and it’s often used to describe objects appearing close together or having a minimal separation. It can also refer to objects having similar ecliptic longitudes or equatorial right ascensions.
This month there are several interesting conjunctions, beginning with a close encounter between the Moon and Mars. As Mars is approaching opposition, it currently appears bright in the sky. At 04:00 BST (03:00 UT) on 6 September, mag. –1.9 Mars appears due south, and 1.3˚ from an 86%-lit waxing gibbous Moon. As the morning progresses, the apparent distance between both worlds reduces so that by 06:00 BST (05:00 UT) they appear 0.6˚ apart. If the weather is clear, try to stay with them into the day. At 07:00 BST (06:00 UT) with the Sun up, binoculars or a telescope should still be able to show Mars just 11 arcminutes north of the Moon’s northern limb.
On 14 September, the apparent movement of the Moon will have it located further east and close to the brilliant planet Venus. Shining at mag. –4.0, Venus will be hard to miss in the early hours. At 05:00 BST (04:00 UT) the 14%-lit waning crescent Moon will sit 3.8˚ north of Venus. As an added bonus, just before the morning dawn, look out for the stars of the Beehive Cluster 1.6˚ south of the Moon (see opposite).
On 18 September, there’s a tricky meeting between a 2%-lit waxing crescent Moon and mag. 0.0 Mercury. You’ll need a flat western horizon for this pair though as they will be extremely low.
On the 24th and 25th, it’s the turn of the gas giants Jupiter and Saturn to get a visit from the Moon. Both planets themselves are coming together for what’s known as a ‘Great Conjunction’, which is set to occur on 21 December. Currently they appear slightly under 8˚ apart. On the evening of 24 September, both planets form a bent line with a 57%-lit waxing gibbous Moon, located 6.5˚ west of mag. –2.3 Jupiter at 21:00 BST (20:00 UT). The next evening, the now 68%-lit Moon will appear further to the east, forming an equilateral triangle with Saturn and Jupiter.