Page 91 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO SEE

PLANETARY ORBITS

Pluto (dwarf planet) Eris (dwarf planet)

Neptune

Uranus

Mercury
Venus
Earth
Mars
Ceres (dwarf planet)

Jupiter
Saturn

The Sun

moves at an average of of just 3.4km/s (12,240km/h). means it shifts considerably actual distance from the Sun,
29.8km/s (107,280km/h), All this speed, or lack of it, day by day. the distance they are from your
while Mercury moves at a affects how a planet moves This is what the gravity of eye, and their size, composition
speedy average of 47.4km/s across the night sky as seen the Sun does, but there’s also and colour. The brightest
(170,640km/h) along its orbit. from Earth’s surface. Whereas its light to consider. We see the planet of all as seen from Earth
Compare that with the furthest Saturn crawls around the sky, planets because the Sun lights is Venus, also known as the
current planetary object, Eris, barely moving among the starry them up. Their brightness is due Evening or Morning Star due to
which travels a slow mean speed skies, Mercury’s fast pace to many things, including their the brilliance of its appearance.

SUPERIOR PLANETS Conjunction

All the planets further from the Sun than Earth are called the
superior planets. These don’t present the same problems for
observers as Mercury and Venus, speeding their way round the
Sun, and so they can be visible all night long if they are in the
right place. When any of these planets line up with the Earth
on the far side of the Sun, they are said to be in conjunction, SUN
and are not observable as they rise with the Sun in the day.
The best time to observe the superior planets is when they
are close to Earth. This happens at around the time called
opposition, when another lining-up takes place. This time the Opposition
planet is on the opposite side of the sky to the Sun, so we are
presented with a fully illuminated disc: visually it’s close to EARTH
or at its biggest and brightest. This is the time to get out your
telescope and have a good old eyeball – you’ll be able to
make out the most amount of detail.

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