The Night Sky in April
The full Moon on September 10 is the closest full Moon to the northern hemisphere’s autumn equinox on September 23. This makes it the Harvest Moon for 2022. This title is given because the fuller phases of the Moon appear to rise at approximately similar times from one evening to the next, exhibiting the least difference in rise times for the year. The equinox marks the instant when Sun’s centre crosses the celestial equator, the projection of Earth’s equator into the sky. On September 23 at precisely 02:04 BST, the Sun passes from the northern to southern half of the sky. The northern hemisphere’s night expands at its greatest rate around this equinox.
The Moon hides the planet Uranus on the evening of September 14, an event known as a lunar occultation. Despite appearing dominant to the naked eye, the Moon is actually quite small in the sky with an apparent diameter around half-a-degree. Your little finger at arm’s length subtends an angle approximately one degree. As it travels around the sky the Moon’s half-degree disc hides plenty of dim stars, infrequently covers bright stars and rarely it’ll cover a planet.
The event on September 14 begins at 22:31 BST seen from the centre of the UK, when Uranus is hidden by the Moon’s bright edge. Reappearance occurs at 23:21 BST, Uranus reappearing from behind the Moon’s dark, night-time, edge. Uranus isn’t a bright planet and at least binoculars are required to see it convincingly. If you do try to see the occultation, start looking 15 minutes before each event to be sure you don’t miss them. The Moon is so close to us that it exhibits parallax, shifting position against the background stars when observed from different locations. This can affect the stated times by up to a few minutes either side.
The September night sky offers excellent constellations and objects to look for. As the bright, vibrant stars of summer slowly and inexorably drift west with each passing day, the rapid expansion of night gives them an extended lease of life. The Summer Triangle is an obvious pattern formed from the three bright stars, Vega in Lyra the Lyre, Deneb in Cygnus the Swan and Altair in Aquila the Eagle. It’s visible high in the south at 21:30 mid-month and 20:30 on September 30.
From a dark site, a bright part of the Milky Way can be seen passing south-west through the triangle, following the vertical spine of a large cruciform pattern known as the Northern Cross; the central part of Cygnus. The foot of the cross is marked by Albireo which means “hen’s beak”. If you have a telescope, do try to look at this star as it’s a beautiful double – currently believed to be a line-of-sight pair – a golden yellow primary with an azure blue secondary.
South of Albireo is Vulpecula the Fox, originally known as Vulpecula et Anser, the Fox and Goose. Whatever you call it, it’s really pretty obscure. Further south and east is the small pattern of Sagitta the Arrow which does actually look like a small arrow. South and east of Sagitta is another small but very distinctive pattern known as Delphinus the Dolphin; it resembles a diamond with a tail.