For astronomers, summer turns to autumn on the 22nd of this month (at 20:21 BST precisely in fact), with the autumnal equinox. Now is a good time to take advantage of the darkening evenings for stargazing before the weather turns too cold.
What is an ‘equinox’?
The autumnal equinox occurs every year in September and marks the beginning of autumn for the northern hemisphere.
The Earth sits on its axis at a tilt of approximately 23°, which means that the northern and southern hemispheres receive different amounts of illumination from the Sun (and different intensities) depending on where the Earth is in its orbit, and so we experience seasons.
There are two points in the year however, when the Earth’s axis is neither pointed towards or away from the Sun and the northern and southern hemispheres are illuminated equally – these are the equinoxes. On these two days, in spring and autumn, there will be almostequal amounts of daylight and night throughout the world – hence the name equinox, which comes from the Latin meaning ‘equal night’!
Above: During the equinoxes, both hemispheres receive equal amounts of daylight. (Image not to scale.) (NASA/GSFC/Genna Duberstein)
Stars and Constellations
Pegasus is the winged horse of Greek mythology seen high in the Eastern skies throughout autumn. Pegasus appears upside down in the sky but is easily recognisable for the four stars that make up the body of Pegasus.
Left: The constellation of Pegasus is seen upside down in the night sky – the red lines highlight the asterism (pattern of stars) known as the “The Great Square of Pegasus”. Image taken from Stellarium mid September around 11pm
Aquarius is one of the oldest documented constellations in the night sky, having been recorded in the 2nd century by Greek astronomer Ptolemy. Although it is the 10th largest constellation, it doesn’t have any particularly bright stars, which can make it difficult to see with the naked eye. Happily, the planet Jupiter sits nearby throughout the month, which might help with spotting the “cup bearer” to the Greek gods. Look low to the south-southeast after about 10:30pm (BST) early in the month, with the constellation moving steadily more south over the course of the night and the month.
Right: The constellation of Aquarius, the water bearer and the nearby planet of Jupiter will be visible throughout September. Also nearby is the planet Saturn, in the constellation of Capricornus. Image taken from Stellarium, mid September around 11pm.
Close to Aquarius is one of the closest nebulae to Earth. At 650-lightyears away, the Helix Nebula is a fine example of a planetary nebula that was discovered in the 18th century. Planetary nebulae (despite their name) have nothing to do with planets. In fact, they are the remains of stars, around the same size of our Sun. When lower-mass stars like our Sun die, they become unstable, and their outer layers of gas and dust are ejected into space. Over time, these outer layers will drift away into space and will be reused by future generations of stars. What is left of the stellar core is known as a white dwarf, a very hot star that shines brightly and causes the planetary nebula to glow. This will be the eventual fate of our very own Sun (but not for another 4.5 billion years or so!).
Above: The position of the Helix Nebula, near the constellation of Aquarius, is highlighted by the blue square. Image from Stellarium, mid -September 2021 around 11pm
The Helix Nebula (also known as NGC 7293 or more romantically, “The eye of God” due to its appearance) sits in the constellation of Aquarius and can be seen with binoculars or a small telescope under the right viewing conditions. For the best view, you want the sky to be as dark as possible – so wait until the moon is new and view from a location away from light pollution.
|Above: Images of the Helix Nebula taken by the Spitzer and GALEX Space Telescopes in Infrared (central nebula, seen as green and red) and Ultraviolet (outer areas, seen as blue) light respectively. Image Credit: NASA/JPL-Caltech||Above: Image of the Helix Nebula taken by the Hubble Space Telescope. Image Credit: NASA/HST|
Venus hangs low in the west at dusk, shining brightly. Sometimes known as the ‘evening star’, the planet will set on the western horizon just before 9pm at the beginning of the month, setting earlier as the month progresses.
Jupiter and Saturn appear in the evening sky, fairly low and towards the south; Jupiter (on the left) is the far brighter of the two giant planets. On the evening of 17th September the waxing Moon can be seen between them, but closer to the horizon.
The above planets will be visible to the naked eye – but if you have binoculars or a telescope you may also spot Neptune and Uranus. Neptune reaches ‘opposition’ this month, when it is directly opposite the Sun in the sky from our point of view. This is an excellent time for viewing or even a spot of astrophotography, as Neptune will appear bright and fully illuminated. Uranus appears in the eastern sky in the mornings throughout the month.
Left: Gas giant Neptune reaches ‘opposition’ this month. Image Credit: NASA/JPL
|New Moon||7th September|
|First Quarter||13th September|
|Full Moon||21st September|
|Third Quarter||29th September|
Throughout the year, full moons are often given nicknames based on different months, seasons, and cultures. This month’s full moon is known as the “Corn Moon” or “Harvest Moon” as it is around this time, at the end of the summer season, that crops are gathered. As the Moon rises early and appears particularly bright, it meant that farmers were able to continue with harvesting their crops into the night.