September 2021

This month, Mars is retreating from the evening sky into the sun’s light. Northern hemisphere sky watchers shouldn’t expect to see it. Southern hemisphere observers probably won’t see it either, but they have a slightly better chance.

Mercury is moving into the evening sky for the first half of the month. A waxing crescent moon passes somewhat nearby on the 9^th. After greatest elongation at 26.8° east of the sun on the 14^th, the messenger planet continues moving eastward against the background of stars (direct motion) toward Spica while its apparent separation from the sun shrinks. The planet settles into a position near Spica in the last few days of September.

Venus, also an evening object, starts the month near Spica and passes it on the 6^th. The moon is close to Venus on the 10^th. Passing near Zubenelgenubi around the 22^nd, Venus enters the Libra diamond by the 26^th. It’s almost out of the diamond on the 30^th.

The best views of these evening setters will be from the southern hemisphere, especially from around 20° to 30° south.

Jupiter and Saturn are retrograde (going westward) in Capricornus this month. Jupiter is near Deneb Algedi all month and moving fast enough to see its position has changed if you watch every few nights. Saturn is west of Jupiter. The ringed planet’s motion slows down enough in the last few days of September to be called stationary in almanacs that still use the term. These two planets being in the constellation make September a great month to learn about the stars of Capricornus. The nearly full moon passes Saturn on the 17^th and Jupiter on the 18^th.

Neptune will be at opposition this month. You need a telescope or binoculars to see it. You also have to know exactly where to look. To make sure you’ve seen it, look several times this month and try to see that the faint blue dot has moved. It’ll be nearest a star labeled HIP115953, which is brighter than Neptune and is about 154 light years away. That star is actually at the limit of naked eye visibility for people with very good eyes and very dark skies. The two will be just 1.5′ apart on the 23^rd and 24^th. Neptune’s distance from us this month is about 4.326 billion kilometers.

Earth is at equinox on the 22^nd at 19:22UT.

Notable conjunctions in September:

3^rd Moon and Pollux 3.0°

5^th Venus and Spica 1.6°

5^th Moon and Regulus 4.5°

7^th Mars and moon 3.8°

9^th Mercury and moon 5.9°

9^th Moon and Spica 5.3°

10^th Venus and moon 3.7°

12^th Ceres and Aldebaran 0.9°

13^th Moon and Antares 4.1°

17^th Moon and Saturn 3.7°

18^th Moon and Jupiter 3.7°

20^th Moon and Neptune 3.6°

21^st Mercury and Spica 1.4°

24^th Uranus and Moon 1.2°

26^th Moon and Aldebaran 6.0°

30^th Moon and Pollux 2.7°

The moon begins September in waning gibbous phase. It’s new on the 7^th, first quarter on the 13^th, full on the 20^th, and last quarter on the 29^th.

The moon is at northern lunistice at 25.9° on the 2^nd, goes south of the equator on the 8^th, reaches its southernmost at 26.0° on the 15^th, crosses the equator northbound again on the 21^st, and reaches a second northern lunistice this month on the 29^th at 26.1°.

Lunar perigee is on the 11^th at 368,500km. Apogee is on the 26^th at 404,700km.