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The Sky at Night for September 2016

Events in DARK text happen during darkness. LIGHT text during daylight, i.e. we can't see them.

Note: Times are calculated for Liverpool and will vary slightly across the UK

Planet Date Rises ? Transits ? Sets ?
Planet Event ? Date Time Sun Angle ? Each month, we identify the best constellations to be seen from the UK between 9pm and 10pm.

This month we'll be looking for the autumn constellations of Aquila, Capricornus, Cygnus and Delphinus,
which can be found towards the south in the September night sky.

Night Sky in September

Aquila - is named after the Latin word for an 'Eagle', and was seen by many ancient civilisations to represent a large-winged soaring bird of some type. It is the 22nd largest of the 88 modern constellations, and its main star, Altair, is the twelfth brightest star in whole the night sky. Its distinct shape, combined with Altair being part of the Summer Triangle (see last month's night sky), makes Aquila an easy constellation for observers to identify. Altair appears so bright because it is relatively close to the Sun, at just 17 light years. The star is well known for having an extremely rapid rotation, with recent measurements suggesting it spins around every 6.5 hours, which is around 100 times faster than our Sun's 25 day rotation period. The result of this rapid spin is that Altair has an oblate shape; that is to say that it's diameter across the equator is 22% greater than the diameter between its north and south poles, i.e. it bulges out at the equator.

Capricornus - is named after the Latin word for a 'horned goat', and is the 40th largest of the 88 modern constellations. It is also one of the 12 constellations through which the ecliptic passes and is therefore one of the signs of the zodiac. The ecliptic is an imaginary line across the night sky that the planets never stray far from during their orbits of the Sun. It marks out the plane of our Solar System, or alternatively, can be used to indicate the direction of its rotation axis (at right angles). Sadly, Capricornus does not rise very high above the horizon during the night, and with only one bright star (Deneb Algedi), this makes it a rather difficult target to identify. In fact, it is the dimmest constellation in the zodiac, apart from Cancer (see March night sky). Your best chance of spotting it would be to first locate Aquila (see above) and then track down in a southerly direction from its leftmost "wingtip" until you come to a brightish star. That would see you at the "horns of the goat".

Cygnus - is named after the Latin word for a 'swan', but is also referred to by some as the Northern Cross due to its distinct shape. It is the 16th largest of the 88 modern constellations and contains 6 bright stars, the brightest of which is the giant star, Deneb, that marks the "tail of the swan". Deneb is also one of the three stars that make-up the summer triangle (see last month's night sky). Its resemblance to a swan flying south along the Milky Way makes Cygnus one of the most easily recognisable constellations. With Altair (see Aquila above) and Deneb appearing to be about the same brightness, it's easy to think that they are about the same distance away. In reality, Deneb is actually 100 times more distant, and only appears so bright because it pumps out around 4,600 times more light than Altair. Deneb is so bright because it is a type of star known as a blue supergiant, whose size is around 200 times larger than that of our Sun. Supergiants are stars in the final stages of their lives, and as such, it's likely that Deneb will become a supernova in the next couple of million years - which could cause significant problems for people living on Earth when it goes off.

Delphinus - is named after the Latin word for a 'dolphin', and is a rather small constellation - 69th largest of the 88 modern constellations. With no bright stars to talk of, observers will have to locate the adjacent constellation of Aquila (see above) to be in with a chance of tracking across to a group of stars that make up the shape of a leaping dolphin. The main star, known as Sualocin, is actually the major component of a multiple star system that contains 6 stars. Observations suggest that around half of the stars in our Solar System are part of multiple star systems, however, 6 component systems remain pretty rare. The presence of so many gravitational attractors in a system would make it very difficult for planets to form.