Page 93 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO SEE

MERCURY This small world is a real challenge to observe for a variety of reasons. It’s
a fast mover, travelling around the Sun four times quicker than Earth, so
don’t expect it to hang about in any part of the sky for very long – it only
Mean distance from the Sun: stays around one week at a time.
58 million km
Mercury’s orbit is a fairly eccentric oval shape, and it’s on a bit of a
Rotation period: 59 days tilt too, which means there are better times to view it than others: spring
Orbital period: 88 days evenings and autumn mornings. If that’s not tricky enough, you only have
Diameter: 4,900km
Gravity (Earth=1): 0.38 a relatively short observation window on any day you choose to look, as
Mercury never goes very far from the Sun.
Surface temperature:
In spring, start looking for the planet 30 minutes after sunset, after
350°C (day), –170°C (night) which point you’ll have about another 45 minutes to see it. Autumn
Number of moons: 0
gives you a longer view, and you can start looking about 1 hour and 45
minutes before sunrise, but that does mean getting up exceedingly early.

Mercury and Venus are worlds apart in that Venus is easy to see most VENUS
of the time. The brilliance of its appearance makes for one of the fi nest
naked-eye views in the evening or dawn skies, especially if the crescent Mean distance from the Sun:
Moon is nearby. 108 million km
Because the ‘Cytherean’ (as opposed to the strictly incorrect adjective
‘Venusian’) orbit is slower than Mercury’s, it can be visible for months on Rotation period: 243 days
Orbital period: 225 days
end, and sometimes for up to three hours after sunset or before sunrise. Diameter: 12,100km
When Venus is at its brightest, it becomes the third-brightest object in
the sky, only beaten by the Moon and the Sun. This is caused by sunlight Gravity (Earth=1): 0.903
Surface temperature:
refl ecting off its bright white carbon-dioxide clouds, and has led to 480°C
Venus being called the ‘Evening Star’ or ‘Morning Star’ depending on Number of moons: 0
when it appears. Venus can come very close to Earth, plus it’s rather big,
meaning that it’s a good target for binoculars, through which you can
easily see its larger phases.

MARS Our fi nal main terrestrial, or rocky, planet differs from Mercury and Venus
in that its position in the Solar System affects the way in which it is visible.
Mars orbits further from the Sun than the Earth and can be ‘up’ from
Mean distance from the Sun: sunset until sunrise.
228 million km
This happens when the planet is at opposition, when Mars is on exactly
Rotation period: 24 hrs 37 min the opposite side of the sky to the Sun. The Sun, Earth and Mars are lined
Orbital period: 687 days up in space, with Earth in the middle. This leads to the planet being at its
Diameter: 6,800km
Gravity (Earth=1): 0.380 highest and brightest at midnight, as this is when Mars is near its closest
Surface temperature: –23°C point to Earth. The weeks around opposition are often when the colour of
the Red Planet becomes truly apparent.
Number of moons: 2
This is the best time to view Mars with a telescope. You’ll be able to see
lighter, pale-reddish areas, the bright white of the ice caps, and darker
patches, which it used to be thought were Martian ‘cities’. Oppositions of
Mars happen once every 26 months, with the last one in March 2012.

CERES – DWARF PLANET

After you pass Mars there are hundreds of thousands of smaller objects in an area commonly known as the
Asteroid Belt. The fi rst one discovered was thought to be a new planet – its name was Ceres and it turned
out to be the largest of these objects. Unfortunately, it was still rather small, which meant it didn’t keep its
planetary status for long. Then, in 2006, its classifi cation changed again when the International Astronomical
Union gave Ceres the status of dwarf planet. As it’s such a small object, you’ll need binoculars to fi nd it.

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