Page 90 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

THE PLANETS

PART 1 INTRODUCTION

SCIENTISTS USE SPACE PROBES TO STUDY THE OTHER MEMBERS
OF THE SOLAR SYSTEM, YET THERE’S STILL SOMETHING SPECIAL
ABOUT SEEING A PLANET WITH YOUR OWN EYES

Out there in space is a pretty recognised as planets and fi ve get to that, let’s take a moment
ment
PLANETS average star about halfway objects classed as dwarf planets, to examine, compare and
IN MOTION through its life, shining away including Pluto, Eris and Ceres. contrast the planets some more.
more.
with a nice even temperament Pluto lost its planetary status The Sun’s gravity ‘well’ is s
DAY – our Sun. I can say all this in 2006, after other similar (and immense – imagine a great
The time it takes for a planet
na
to spin once on its axis because, after all, we wouldn’t some larger) objects were found bowling ball creating a dip in a
e like
relative to another object. be here if our star wasn’t so where it orbits. But to meet trampoline. The planets are like
For Earth, relative to the calm and collected. today’s defi nition of a planet, marbles rolling along inside this
this
Sun, the day is 24 hours The Sun isn’t actually as well as being rounded by its dip around the bowling ball Sun.
Sun.
long. But relative to the stars stationary, but travels around own gravitional force and in The closer you are to the Sun,
un,
it’s 23 hours 56 minutes (or orbits) the centre of our orbit round the Sun, a body has the stronger its pull of gravity
ity
and 4 seconds because
the Sun moves against this Galaxy about every 250 million to have cleared its orbit other and the faster you have to move
fi xed background. years – known to those in the objects its size, which Pluto to keep from being pulled into
ORBIT trade as one galactic year. Of hasn’t done . solar destruction. By the same
A planet’s orbit is the course, the Sun is not alone on logic, the further you travel
circular or elliptical path it its journey; its gravity pulls OUR SOLAR SYSTEM from the Sun, the less its pull
makes around the Sun. a rag-tag bunch of comets, All the planets move in the same of gravity and the slower you
YEAR asteroids, moons and planets anticlockwise direction around travel in your orbit.
The time it takes for a planet along for the ride. All together the Sun, if we take Earth’s north Earth takes one year to
to orbit once around the these take on the title of the pole as an arbitrary reference travel around the Sun, but
Sun. One year for Venus Solar System. of ‘up’. Several of them are big closer Mercury takes only 88
is 224.7 Earth days, while
one year for Neptune is The number of planets there enough and close enough to Earth days. Looking at the
164.8 Earth years. are has changed over time. Earth to be a worthwhile target comparative speed of each
Currently there are eight bodies for your telescope, but before we planet, we fi nd Earth

INFERIOR PLANETS Superior conjunction

The two planets that are closer to the Sun than Earth are Mercury
and Venus, and together they are known as the inferior planets. Eastern
Due to the nature of their orbits, the best time to observe them is elongation Western
when they are at their farthest angular distance from the Sun, a Inferior elongation
position astronomers call elongation. conjunction
At these times, the planets are only half lit by the Sun, but after SUN
this they swing back into the solar glare, where they become less
visible. For example, when Mercury and Venus are at eastern
elongation, they set after the Sun in the evening; when they’re at
western elongation they rise before the Sun in the morning.
PAUL WOOTTON X 2, NASA during their orbits: when it, Earth and the planet are lined up.
The Sun interferes with our views of the inferior planets twice
When a planet is between Earth and the Sun, it’s at inferior
conjunction; when it’s on the far side of the Sun, it’s said to be
at superior conjunction.

90 skyatnightmagazine.com 2012 EARTH