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Astronomy and Space articles
by Martin George of the Launceston Planetarium
16 November 2013
The Upside-Down Cross
Of all the 88 star patterns, or constellations, in the sky, the Southern Cross is certainly the
most famous for us in Australia. It is a compact but quite prominent group of stars that
really does look like the shape after which it is named.
I was asked recently why we don't see the Cross in the evenings at this time of the year.
However, we do indeed see it, as the Cross never goes below our Tasmanian horizon! I
immediately asked whether there were tall buildings or trees to the south of my friend's
house - and indeed, as I expected, the answer was yes.
From any latitude other than the equator, there is a region in the southern sky if one is
south of the equator - or the northern sky, if one is north of the equator - that is
permanently above the horizon. For our latitude, this region is quite large, with a
diameter in the sky equal to about four hand spans held at arm's length and centred on a
point nearly half way up the southern sky, called the South Celestial Pole.
The Southern Cross, and the two famous stars called the Pointers, lie within this region,
so the Cross never sets and is visible on any clear night.
However, it is indeed very low down in the evening sky during November, and by the time
the sky is completely dark late in the month, it is at its lowest point in the sky - due south,
and upside down.
The relative positions of the stars don't change as the stars appear to move around us.
This apparent movement is, of course, because of the Earth's rotation. We turn once
every 23 hours, 56 minutes and 4 seconds, and the stars appear to turn around us at the
same rate.
The directions to look for the star patterns, though, do depend on the time of night and
the time of year. During the mid evening in May, by contrast, the Cross is high in our sky
and almost upright.
It's easy to keep track of these apparent motions, and you don't need a computer! One of
my very first astronomical tools was a device known as a planisphere, with which you can
'dial up' the time of night and the date, and a window will reveal the stars that are visible,
and the directions to look. They are obtainable in many good bookshops and from
binocular and telescope suppliers for a quite modest price.
Incidentally, not only is the Cross very low down in the sky on November evenings, but so
is the glow of the Milky Way. It's not nearly as prominent when so low in the sky, partly
because some of its light is lost during its passage through our atmosphere. Therefore,
Astronomy and Space articles
by Martin George of the Launceston Planetarium
16 November 2013
the evening sky does not look nearly as spectacular now as it did during winter and
spring.
If you get up during the small hours of the morning, you will see the Cross to the left of
where it was mid-evening and higher in the sky - showing you that after reaching that low
point, it climbs again as the Earth rotates!
Enjoy your stargazing on these milder evenings.
Article by Martin George, Launceston Planetarium, QVMAG.
Reproduced with permission of the Mercury newspaper.