28 September 2019 | New Scientist | 51What you need
A clear night sky
For next week
A star chart or phone with
a stargazing app will be helpful
but not essential
Next in the series
1 Model the equinox
2 Find the North Star
and Southern Cross
3 Test your area’s light
pollution
It’ll help you plan
stargazing trips
4 Identify the craters
of the moon
5 Orion and Sirius: how
to star-hop
6 Planet spotting: Mars,
Mercury and Uranus
7 Taurus and the zodiacal
constellations
IMAGINE there is a shell encasing
Earth, and every star is painted on
it. The shell constantly rotates, so
the stars rise and set like the sun.
But one bright star doesn’t move:
Polaris, the North Star.
That is because its position is
directly above the North Pole and
therefore Earth’s axis of rotation.
This means that if you are north
of the equator it can be used for
navigation, as it can always be seen
at due north at an angle in the sky
equal to the latitude you are seeing
it from.
The shell of stars doesn’t make
a full rotation each day, though.
If you looked up at 7 pm one day,
and again the next, the stars would
be shifted by four degrees. This is
because we measure time in solar
days – the time it takes Earth to
rotate with respect to the sun’s
position – rather than sidereal
days, the time it takes Earth to
rotate with respect to the stars.
The two are different because
Earth is also orbiting the sun as
it rotates, meaning a solar day is
slightly longer and the stars rise
4 minutes earlier each day.
Polaris is so bright despite
being 433 light years away because
it is a triple star system. A yellow
supergiant called Polaris Aa orbits
smaller Polaris Ab, and the pair
orbits another star, Polaris B. It is
part of the constellation Ursa
Minor, or the Little Bear.
You can find Polaris using the
constellation Ursa Major, part of
which is the Plough, or Big Dipper.
This moves around the North Star,
but two of its stars always point to
Polaris: Dubhe and Merak. Dubheis a multiple star system 123 light
years away, and Merak is a subgiant,
meaning it has used up all its
hydrogen and is now cooling.
To find Polaris, draw a line
from Merak to Dubhe, and then
extend it five times the distance
between them. The line ends at
the North Star.
There is no bright star over the
South Pole, but you can find due
south using the Southern Cross,
part of Crux, the smallest of the
88 official constellations.
The Southern Cross has four
main stars: Alpha Crucis, Beta
Crucis, Gamma Crucis and Delta
Crucis. These are easy to spot as
they have the same brightness.Plus there are two bright stars in
the constellation Centauraus that
point to it: Alpha Centauri and
Beta Centauri. Alpha Centauri is
notable for being the closest star
system to Earth, at just 4.37 light
years away.
Once you have found the
Southern Cross, draw a line from
the top star in the cross to the one
at the bottom. Extend it 4.5 times
the distance between the stars and
you are at the south celestial pole
and are looking due south.
Next week, I will show you how
to work out the light pollution
level in your area and so gauge
how many planets and stars it
will be possible to see. ❚Stargazing at home Week 2
Navigate by the stars
Essential for every stargazer is the ability to find north and south
using the stars. Abigail Beall shows howPuzzles
Quick crossword,
a darts puzzle and
the quick quiz p52Feedback
Political seance and
food for thought: the
week in weird p53Almost the last word
Water movements
and battery life:
readers respond p54The Q&A
Sam Rogers on
designing and flying
a jet suit p56Picture of the week
Our pick of your
women in STEM
themed photos p53The back pages
Abigail Beall is a science writer
in Leeds, UK. This series is
based on her book The Art of
Urban Astronomy @abbybeall
Stargazing at home online
Projects will be posted online each week at
newscientist.com/maker Email: [email protected]MERAKTHE BIG
DIPPER,
OR PLOUGHPOLARISDUBHE