The Night Sky in September
The Northern Hemisphere’s autumn equinox occurs at 02:54 BST on Sept 23. This is the instant in time when the centre of the Sun’s disc moves south across the projection of Earth’s equator in the sky; a great circle known as the Celestial Equator.
As far as the average human is concerned, Earth’s rotational axis remains fixed with respect to the stars as we orbit the Sun. Our planet’s southern pole tilts towards the Sun in December and the northern pole towards it in June. During late March and late September, the orientation is such that the Earth is effectively sideways on to the Sun. This is when the equinoxes occur, a time when the length of day and night is more or less equal.
The closest full Moon to the Northern Hemisphere’s autumn equinox is the Harvest Moon. This year the full Moon on Sept 25 takes this title. The name is given because of the Moon’s propensity to illuminate fields for harvest. The reason why this particular full Moon is singled out is that at the time of the September equinox, the ecliptic creates its shallowest angle with the eastern horizon around the time of sunset – that’s when the full Moon would be rising. The ecliptic is the plane of the Earth’s orbit projected as another great circle on the sky. This indicates the apparent path of the Sun against the background stars over the course of a year and to an approximation, the main planets and the Moon always appear close it.
Each day the Moon moves approximately 12 degrees east along the ecliptic, that’s 24 times its own apparent diameter. The shallow ecliptic angle at moonrise in September means the Moon loses little altitude each consecutive night resulting in similar rise times for the fuller phases of the Moon. This provides a natural bright light to assist with harvest collection.
When the bright Moon is out of the way, the stars of autumn are now beginning to move centrestage. The main pattern visible is the Great Square of Pegasus. Despite the pattern’s name, the star in the northeast corner, formerly Delta Pegasi or Sirrah, was re-assigned to neighbouring Andromeda as Alpheratz in 1928. To be doubly pedantic, it’s also worth noting that none of the sides of the square have equal lengths!
Returning from the stars to the Solar System, comet 21P/giacobinizinner is well placed during September. This binocular comet tracks southeast through Auriga the Charioteer, passing close to the open cluster Messier 37 on the nights of Sept 9-10 and 10-11. Coincidentally, the comet will be at its closest to Earth at this time, passing us by 58 million kilometres. This is when the comet will appear brightest but its subsequent dimming throughout the rest of September isn’t severe and, given clear skies, it should be relatively easy to keep track of. Another nice encounter occurs on the nights of Sept 14-15 and 15-16 when 21P passes close to the open cluster Messier 35 in Gemini.
Solar System objects passing near deep sky objects has little scientific value but nevertheless holds a certain fascination. Mars remains bright and obvious. On Sept 16 it will be at perihelion, the point in its orbit where it is at its closest to the Sun. It will be interesting to see whether this closer proximity promotes further dust storm activity in its thin atmosphere. A major dust storm took place in June, an event which obscured many of the planet’s dark surface features.