21 September 2019 | New Scientist | 51What you need
Cardboard
Two balls
Wire
Sticks
Glue and scissors
For next week
A clear night sky
Next in our
7-week series
1 Model the equinox
2 Find the North Star
and Southern Cross
Learn to navigate
by the stars
3 Test your area’s light
pollution
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
EVER looked up at the night sky
in wonder? Ever wished you could
spot Mars or navigate by the stars?
Then our new astronomy series is
for you, wherever you live in the
world and even if you are in a light-
polluted city. Better still, no special
equipment is required.
Crucial to making sense of the
night sky is understanding Earth’s
movement around the sun. As an
equinox falls on 23 September, I’m
going to kick off the series with a
simple model to explain what it is.
There are two equinoxes in the
year; they are the points when the
lengths of day and night are nearly
equal over the world. The one in
September is when the southern
hemisphere begins to tilt slightly
more towards the sun and spring
begins. In the north, it marks the
start of autumn, or fall.
The equinox isn’t a day-long
event, however. It is the exact
moment when the sun crosses
the plane of Earth’s equator, which
varies according to your latitude.
The model we are making won’t
be to scale, but that’s OK for our
purposes. Start with a circular
piece of card. Cut another piece
into a rectangle the same length
as the circle’s radius, like the one
in the picture. Then pierce a hole
at one end of your rectangle and at
the centre of your circle and push a
stick through both. Glue a ball on
top to represent the sun.
Next, bend a piece of wire to
23.5° – that is the angle of Earth’s
axis of rotation – and stick it into
the other end of your rectangle.
Pierce the poles of your “Earth”
ball and stick the wire through.Now move your Earth around
the sun, keeping your polar wire
pointing in the same direction at
all times. You’ll come to a point at
which the northern hemisphere
tilts towards the sun. This is the
June solstice, marking the
northern summer.
As you move your Earth further
round its orbit, it will reach a point
where both hemispheres face the
sun the same amount – this is
where we are now, the September
equinox. When the south pole tilts
most to the sun, you have reached
the December solstice, and the
March equinox comes when both
sides are equal again.
The tilt of Earth is also moving,in a process called precession. To
simulate this, hold the end of the
wire protruding from Earth’s north
pole and make it trace a small circle
(see picture). This rotation takes
25,772 years to complete, meaning
that in around 13,000 years’
time, summer in the northern
hemisphere will happen when
it currently experiences winter.
It also means the North Star will
change, because the north pole
will no longer be pointing in the
same direction. Instead of Polaris,
the bright star Vega will be the
north star. Which brings us to next
week, when we’ll be using what we
have learned to find the North Star
and the Southern Cross. ❚Stargazing at home Week 1
Earth’s celestial moment
It’s almost the September equinox, making this the perfect time
to kick off our exploration of the heavens with Abigail BeallPuzzles
Cryptic crossword,
an egg puzzle and
the quick quiz p52Feedback
Elongated eels and
naming names: the
week in weird p53Almost the last word
Lightning effects and
biscuit/cake duality:
readers respond p54The Q&A
Randall Munroe
on the creative use
of science p56Picture of the week
Our pick of your
future-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
ANGLE OF EARTH'S TILTPRECESSION23.5^0EARTH SIDE-ON TO
SUN AT EQUINOXStargazing at home online
Projects will be posted online each week at
newscientist.com/maker Email: [email protected]