Autumnal Equinox
September is a transitional month as the long summer days here in the North start to shorten and we move towards darker skies and cooler nights. Also, the constellations begin to change from the summer collections of The Summer Triangle, Scorpius, and Leo – to be replaced with the winter skies of Auriga, Gemini and of course – Orion. The highpoint of the month is the occurrence of the Autumnal Equinox on Saturday 23rd September, but what exactly is the equinox, why does it occur and what does it mean for Astronomy?
The science behind an equinox
The Earth rotates on an axis, and that axis is tilted by approximately 23.5 degrees, which means the sun shines at different angles depending upon where the observer is on the Earth, and where the Earth is in its orbit around the sun, this is why we enjoy seasons. In the Northern hemisphere we tilt towards the sun during the Summer (meaning longer summer days) and away from the sun during Winter (meaning longer winter nights). At two points in the year the Sun will shine directly above the planet’s equator, meaning the sun will illuminate the northern and southern hemispheres equally. These are known as the equinoxes and will result in days and nights of the same length. Watch this video from the National Geographic for more:
Whilst the September equinox is called the Autumnal equinox, does this mean Autumn has started? Well not quite, and it depends who you ask. There are three ways to decide when autumn is: astronomical, meteorological and phenological. Let's have a look at each.
Astronomical
Astronomically, each solstice and equinox is used to mark the start of the season, so Astronomically speaking autumn will begin on Saturday 23rd September, at the exact moment of the Autumnal equinox. However, there is disagreement between those who see the equinox or solstice as the start of the season, and those who hold that it represents the middle of the season.
Meteorological
Meteorologists divide seasons into periods of three whole months based on average monthly temperatures, with summer as the warmest and winter as the coldest. On this basis, the start of autumn for meteorologists will be the 1st of September and it will last for the whole of September, October, and November. Meteorologists pay no regard to the equinoxes or the solstices.
Phenological
Phenological seasons are based upon a range of ecological and biological factors and indicators, such as the leaves falling off the trees, bird migration and mammal hibernation. All these events are influenced by weather and climate, which means that the seasons can move around more, for example if daffodils flowering determine the start of spring, this can happen much earlier in a year with a relatively warm winter.
So, depending upon how you decide to measure it, Autumn will either begin September 1st, September 23rd or when you see enough natural signs to make it real to you, just take your pick. Tell us in the comments below, when does autumn start according to you?
Planetary Observations
Let’s move on to look at what’s in the sky this September, what you can see and when. The Moon
First off here are the moons phases along with the dates.
• Last Quarter – 6th September
• New Moon – 15th September
• First Quarter – 22nd September
• Full Moon – 29th September
September’s full moon, which will almost be a Supermoon, (see last month's Stella Cartography for more info on that) is known as the Harvest Moon, as it is the full moon closest to the autumnal equinox. It was named because in the northern hemisphere, the Harvest Moon rises very soon after sunset, providing plenty of bright light for farmers harvesting summer crops. The Harvest Moon is also a point of celebration for cultures across the world including in China where it is the known as the Mid-Autumn Festival, or Mooncake festival. On the full moon night of the eighth lunar month, people gather with friends and family to admire the brilliant full moon while eating mooncakes, a delicious rich pastry filled with sweet bean paste or lotus seed paste.
Mercury
Mercury transitions into a morning planet this month and, by the middle of September, is high enough in the east before sunrise that you should be able to get a reasonable view. As the month progresses Mercury will steadily brighten as it continues through its orbit, and we see more of the light side of the planet. Best mornings to view will be between 15th and 25th September, approximately an hour before dawn in the East. If you are looking on the 15th, find the bright star Regulus in the constellation of Leo, Mercury will be a little bright star below that towards the horizon.
Venus
Venus really takes off in bright way during September, providing you can get yourself up early enough to enjoy her show. She is very much a morning planet now, and much like Mercury she will continue to brighten throughout September, as more of the lighter part of the planet becomes visible to us. Best days to catch a glimpse would be the 15th of September, and to be honest you’ll be hard pressed to miss her due to her brilliance. Look East and follow the instructions above to find Mercury first… then look up and to the right beyond Regulus, you’ll spot her shining like a diamond in the constellation of Cancer the crab.
Mars
Mars is having the worst viewing conditions right now as it is passing behind the sun, relative to us here on Earth, so it’s just not visible really. Best to wait until Spring 2024 to catch sight of the Red Planet again.
Jupiter
Carrying on from August viewing, Jupiter is rising earlier and earlier, and by the end of the month will be well positioned for viewing before 9pm. It is by far the brightest thing up there currently so almost impossible to miss. But for a great chance to see the King of the Planets, look due East at 11:30pm on 15th of September, you honestly will not be able to miss him as he sits in the void between the constellations of Ares the ram and Taurus the bull – nestled to the left of Cetus the whale. Another key to find this planet will be the Pleiades star cluster which will sit to the left of the planet and slightly under it.
Saturn
Saturn reached opposition in August and is now an evening object. It is as bright as it is going to get this year, with a magnitude of about 0.4, so easily visible by the naked eye. The best way to track down Saturn is to look South on 15th of September, at about 11pm. Saturn is sitting in the constellation of Aquarius and is by far the brightest star looking south. It’s about 40° high at its highest, which is roughly midway between the horizon and overhead.
Uranus
Grab those binoculars (or telescope if you’re lucky enough to have one) you’ll need it to see this one, but there is a great opportunity to spot Uranus (no jokes please) near to the Moon on 5th September from midnight onwards. Find the moon (you shouldn’t need any help with that), Jupiter will be the bright star to the right, Uranus will be a very pale green star below and to the left of the moon, you will need dark skies and a good pair of binoculars to see him however.
A great opportunity to find Uranus easy on 5th September after midnight.
Neptune
Neptune reaches opposition on 19th of September and is now rising in the east at sunset and, if you have a good telescope, visible all night for the rest of 2023 through to February of 2024. On the day of opposition, 19th September, look for the circlet of stars in the constellation of Pisces the fish looking South-East, Neptune will be slightly below that (good luck!).
Next Month’s Preview
Next month's Stellar Cartography will be looking at the astronomical events of October 2023, including an annular solar eclipse, and a partial lunar eclipse. Until then, happy star gazing!
What did you think of this edition of UFP Stellar Cartography? Let us know in the comments below.
WRITTEN BY WoorLord
EDITED BY Solace
IMAGES SOURCED FROM Moongiant.com - Jodrel Bank - Pintrest.com - Wallpaper-mania.com
