Out & About Autumn 2019

Out&About leisure

STAR GAZING

Newbury Astronomical Society’s NIC FLEET presents a round-up of some of the night sky highlights to look out for during the autumn months

light years distant and almost twice as massive as our sun. It rotates extremely fast – in only nine hours compared with 25 days for the sun – so its shape is flattened at the poles. Altair also lies in the line of the Milky Way. See if you can trace the shape of our home galaxy all the way down from Deneb to Altair. Transit of Mercury

Jupiter – the king of the planets

The Summer Triangle

image Stellarium

The giant planets Jupiter and Saturn will be prominent low in the southern sky at the beginning of September. On September 5, the first quarter moon is just above and to the right of Jupiter – and on September 8 it will be slightly below and to the left of Saturn. A pair of 10x50 binoculars, mounted on a steady tripod, should enable you to spot Jupiter’s four brightest moons looking like stars lined up on either side of the planet. The Galilean moons are named in honour of the Italian astronomer Galileo Galilei, who first observed them in January 1610. With a small telescope you can watch the dance of the moons as they orbit Jupiter, disappearing into the planet’s shadow and reappearing from behind it, and even watch as the dark shadow of a moon occasionally crosses the planet. Free planetarium programs such as Stellarium, or a phone app, will tell you which moon is which and give you the times of disappearance and reappearance. A small telescope will be needed to show Saturn’s rings and its largest moon, Titan.

If you look south as the early autumn sky darkens, three bright stars will be the first to emerge from the twilight glow. Two will be almost overhead, while the third, at the point of the triangle, will be about halfway to the horizon. Although these stars belong to different constellations, they are collectively known as the summer triangle (although the autumn triangle might be more appropriate). The faintest of these stars is called Deneb, although it’s the brightest in the constellation of Cygnus, the Swan. It may appear fainter than the other two, but it’s actually a white supergiant star (one of the largest known) around 200,000 times as bright as our sun. It only seems to be fainter because it’s around 3,500 light years away. The rest of Cygnus forms the shape of a cross stretching along the line of the Milky Way – so it’s also sometimes known as the Northern Cross. At the head of the Swan, or the base of the Cross, is a star called Albireo. This is, in fact, two stars close together, and a small telescope will show their beautiful contrasting colours of blue and amber. The star Vega – about a handspan at arm’s length to the right of Deneb – is only about 25 light years from our sun and is the fifth brightest star in the night sky. It’s about 2 and a half times the radius of our sun and about 50 times as bright. It also has the distinction of having been our pole star in about 12000 BC. Because Earth’s axis wobbles like a spinning top, the position of the north celestial pole changes very slowly. In roughly 12,000 years’ time it will have gone full circle and Vega will be our north star once again. Altair is the closest star of the three, 17

Most of the planets in our solar system orbit the sun in roughly the same plane, so occasionally the planets between us and the Sun (Mercury and Venus) pass directly in front of it. Astronomers call these passes ‘transits’ and during one, with the right equipment, we can see the tiny disk of the planet silhouetted against the sun’s huge fiery face. Venus last passed directly between Earth and the sun in June 2012, but there won’t be another transit of Venus until December 2117. The next Mercury transit will take place on November 11, 2019, but after this you’ll have to wait until November 13, 2032. Providing it’s clear on November 11, it will be possible to watch Mercury travel about two thirds of the way across the sun, starting from about 12.30pm and finishing when the sun sets. Mercury’s disk will only be about 12 arcseconds across – roughly 150 times smaller than the sun – so it will appear as a tiny black dot, too small to see through eclipse viewers. The safest way to observe the transit is to project the image of the sun through a small telescope and onto a piece of card. You must never, ever, look directly at the sun through any type of optical aid as this will seriously damage your eyes. Alternatively, keep an eye on our website for any events we are running in the Newbury area where you can watch the transit safely.

The rings are currently tilted at an angle of 24 degrees to us, but are slowly closing up, so in a few years’ time we’ll see them almost edge on.

newburyastro.org.uk

Pictures: John Napper

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O&A AUTUMN 2019