In a month when half the entries in our Diary concern planets that stand low in our E pre-dawn sky, the highlight is the dramatic conclusion to the Cassini mission at Saturn. Almost twenty years since its launch and thirteen since it arrived at its target world, the spacecraft is due to plunge to its destruction in the Saturnian atmosphere on the 15th.
Saturn, an evening object in Ophiuchus, hovers low in the SSW at nightfall and sets in the SW some 40 minutes after our chart times. It dims from mag 0.4 to 0.5 and stands 4° below-left of the Moon on the 26th.
Jupiter, bright at mag –1.7, may still be glimpsed just above the WSW horizon as the night begins, but by the 12th it sets less than one hour after the Sun and will soon be lost from sight.
The Milky Way arches overhead from the NE to the SW at our map times, flowing through the Summer Triangle (Vega, Deneb and Altair) just to the W of our high meridian. The Pleiades in the E herald the rising of Orion in the small hours.
Venus, mag –4.0 to –3.9, is by far the brightest object in our E sky before dawn. Showing a small dazzling gibbous disc through a telescope, it rises in the ENE at about 03:15 on the 1st and lies 1.2° below-right of the Praesepe or Beehive star cluster (use binoculars) in Cancer on that morning.
Our second brightest morning planet, Mercury, improves from mag 1.1 to –1.1 and rises more than 80 minutes before the Sun from the 6th to the 23rd. Below-left of Venus, it stands 5° or higher in the E 40 minutes before dawn during this spell.
Mercury partners Regulus (mag 1.4) in Leo until the 12th and then the fainter Mars (mag 1.8) as they lie within 0.5° on the 16th and 17th. The waning earthlit Moon joins the show on the 17th when it stands above Venus, Regulus and the Mercury-Mars conjunction. The line-up is even more compact on the 18th, with the Moon just below Regulus, while on the 20th Venus passes just above-left of Regulus. Mars sits 3° below-left of Venus by the month’s end.
September diary
1st 18h Venus 1.2° S of Praesepe
5th 06h Neptune at opposition; 12h Jupiter 3° N of Spica
6th 08h Full moon
10th 13h Mercury 2.9° S of Regulus
12th 11h Mercury furthest W of Sun (18°)
13th 07h Last quarter
15th 13h Cassini impacts Saturn
16th 19h Mercury 0.06° N of Mars
18th 02h Moon 0.5° S of Venus; 06h Moon 0.1° N of Regulus; 21h Moon 0.1° N of Mars
19th 00h Moon 0.03° S of Mercury
20th 00h Venus 0.5° N of Regulus; 06h New moon
22nd 21:02 Autumnal equinox
27th 01h Moon 3° N of Saturn
28th 04h First quarter
* Times are BST
