30 November 2019 | New Scientist | 51The back pages
SUNMERCURY28 ɾ VENUS47 ɾEARTHStargazing at home online
Projects will be posted online each week at
newscientist.com/maker Email: [email protected]Abigail Beall is a science writer
in Leeds, UK. This series is
based on her book The Art of
Urban Astronomy @abbybeall
What you need
Binoculars
A good view of the
eastern horizon
For next week
A dark night at
high latitude
Next in the series:
1 Mercury transits the sun
2 How to watch the Leonid
meteor shower
3 Venus and Jupiter
in conjunction
4 Mercury at its greatest
elongation
5 How to see the
Northern Lights
Where to go and
what to look for
6 Find the Andromeda
galaxy
7 How to see Santa (the
ISS) on Christmas Eve
MERCURY passed in front of the
sun two weeks ago in a rare transit
event. This week, we get a chance
to see the planet at night. But you
will have to set your alarm.
Mercury and Venus are known
as inferior planets, because they
orbit closer to the sun than Earth
does. On 28 November, Mercury
reached its greatest western
elongation. This is the point in its
88-day orbit of the sun when the
distance between the sun and
Mercury as seen from Earth is
the biggest it gets. This apparent
distance is also known as the
angular separation. In the
following days, the planet dips
a little lower in the sky, but you
should still get a good view.
Seeing the planets depends on
them being as far from the sun as
possible, because when they are
right next to it, the light they
reflect is outshone by the sun
itself. This matters even more for
Mercury than it does for Venus,
because it is much smaller and
fainter than its neighbour, and
so close to the sun. The rocky
planet, with a cratered surface
rather like our moon’s, is less than
5000 kilometres in diameter. That
makes it smaller than Jupiter’s
moon Ganymede, and Titan,
which orbits Saturn.
Mercury’s maximum angular
separation varies between 18 and
28 degrees according to how close
it is to the sun in its highly
elliptical orbit (the absolute
maximum separation is shown
in the illustration). For Venus,
it is between 45 and 47 degrees.
Mercury’s angular separation isIf you are up before sunrise, look out for the smallest planet as it
reaches the spot where it is most easily seen, says Abigail BeallStargazing at home 2 Week 4
Mercury rising in the east
Almost the last word
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on the world p53Feedback
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Jeffrey Hangst
on how to make
antimatter p56smaller because its average orbit is
about 58 million km from the sun,
while Venus’s is 110 million km and
Earth’s is about 150 million km.
Mercury’s proximity to the sun
makes it hard to see most of the
time, with the best viewing
opportunities being just before
sunrise or just after sunset.
While the greatest elongation is
on 28 November, Mercury will be
visible for two weeks after this,
rising 2 hours before sunrise and
reaching 10 degrees above the
horizon 45 minutes before sunrise.
People in both hemispheres
should get a look, but those in
the north will have the best view.
To spot Mercury, try to find theconstellation Libra and the planet
should be there.
Binoculars will give you a better
view, but you can see the planet
with the naked eye. At the time
it rises, Mercury will have a
magnitude of −0.44, so should be
visible to most people with clear
skies regardless of light pollution.
Because it is close to the horizon, it
will shine with a pink hue, like the
setting sun. That is because Earth’s
atmosphere scatters light coming
through it, reducing the amount
of blue light reaching your eyes.
The nights are getting longer in
the northern hemisphere, so next
week is a guide to the best way to
watch the aurorae there. ❚