Page 18 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

HOW ASTRONOMERS DESCRIBE THE POSITION
OF ANY GIVEN OBJECT IN THE NIGHT SKY, SO
STAR THAT YOU KNOW WHERE TO LOOK FOR IT

CO-ORDINATES

USING CHARTS On page 29, we’ll be looking at the brain-defying measuring northward or southward – we call this
units of length in space, and how astronomers 0° latitude. We use degrees (°) because when we
If you’re looking for the star work out just how far apart things like galaxies or locate places on Earth, or on the celestial sphere,
Deneb on a chart, you can stars really are. But you can forget all about that it’s done using angular measurements. Latitude
fi nd it with the following for now. It’s all truly fascinating stuff and great increases as we move round the Earth northwards
coordinates:
for discussions with a cup of tea over the garden or southwards, reaching a maximum of 90°N at the
fence, but there’s no practical benefi t in knowing North Pole and 90°S at the South Pole.
Right ascension (RA)
that Deneb is over 70 times further away than Vega Lines of longitude, meanwhile, start from the
20h 41m 25.9s
when you’re out with your telescope. North Pole and run ‘down’ Earth, crossing the
Declination (Dec.)
The fact is, everything is so far away from us equator and ending at the South Pole. These locate
+45° 16’ 49”
that we can consider it to be the same distance. things east to west on the plane and are also
Clearly, this is not all just This applies as much to a distant galaxy billions of measured in degrees. Here, of course, longitude
nice simple degrees. In lightyears away as it does to the Moon at just a few crosses all the points around the equator (a circle),
declination the ‘ symbol
represents angular (or arc) hundred thousand kilometres. Include man-made amounting to 360° in total. Actually, we move
minutes and the “ represents satellites and you bring the fi gure down to a few westward up to 180° and eastward 180°, but it all
angular (or arc) seconds. hundred kilometres. adds up to 360° in the end.
A degree is a pretty What is the point of assuming everything’s at For the celestial sphere, we throw the whole
large unit on the sky – two one distance? It allows us to describe the position of latitude and longitude Earth grid up into the sky
widths of the full Moon, something, as well as locate that celestial wonder. – it’s a mirror image. There’s no reason why we
in fact! So, 1° is divided This is all done by the power of the ‘celestial couldn’t have used celestial latitude and celestial
into 60 arcminutes and
each arcminute has 60 sphere’. Regardless of distance, we create a longitude as titles, but those who know better
arcseconds – used for super sphere around us onto which everything spacey is decided otherwise. So instead – and I cannot
accuracy or very small projected. You could even include planes or birds if apologise enough – we have ‘declination’ for latitude
things. The + or – at the you so wished. and ‘right ascension’ for longitude.
start shows whether it is in Not only can the location of any object
the northern (+) or southern CELESTIAL LATITUDE be described by its right ascension (RA) and
hemisphere (–).
Right ascension is written The celestial sphere works much like mapping on declination (Dec.), but when you get down to
as hours, minutes and Earth. As you’ll remember from geography lessons looking at the paths of objects across the celestial
seconds – as in regular at school, to locate something down here we use sphere, you can understand some interesting things
time, not the arcseconds latitude and longitude. The equator is the most about how Earth moves, works and relates to the
of declination. This is why, famous line of latitude and is the starting point for rest of space.
for example, minutes are
shown as m (time variety)
and not ‘ (arc variety). So,
one hour in RA describes MEASURING
the movement of the sky due
to Earth’s spin over an hour THE SKY
– which is 15°, because 15°
x 24 (hours) is 360°, and 1˚
that’s all the way round over Hands and fi ngers are very useful for getting
the course of a day. to grips with sizes in the sky. Assume the
Yes, it’s bonkers! following are held out at arm’s length: an
outstretched hand spans around 22°, which
Needless to say, star charts
STEVE MARSH, PAUL WOOTTON there’s no need to convert is around 10°, while your thumb covers 2°.
is close to the length of the Plough. Your fi st
are all divided up nicely, so
We seem to get the impression the full Moon
anything – just plot the
is a lot bigger in the sky than it really is, but
position and there will be
amazingly, the end of your little fi nger (again,
Deneb, in the tail of Cygnus,
at arm’s length) is around 1°, which means it
the Swan.
18 skyatnightmagazine.com 2012 will cover the full Moon twice!