Among the stars in our sky, there are some that appear to move relative to the constellations. These were noticed by every ancient civilisation and are now known to be planets orbiting the sun; in fact, the word 'planet' comes from the ancient Greek word for 'wanderer'. Mercury, Venus, Mars, Jupiter and Saturn are all bright enough to see without a telescope.
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As these planets move around the sun, they also appear to move through our sky. However, since Mercury and Venus are closer to the sun than Earth, they never stray far from the sun in our sky. For this reason, they can only be seen just after sunset or just before sunrise. Mars, Jupiter and Saturn are further from the sun and can sometimes be seen for the entire night, depending on the time of year. The planets all orbit the sun on elliptical paths, roughly confined to the same flat disc.
As a result, the planets and the sun all appear roughly on the same line across the sky, called the ecliptic. Mercury and Venus constantly move back and forth along this line either in front or behind the sun.
The outer planets predominantly move west-east along the ecliptic, although occasionally they switch direction as Earth 'overtakes' the slow outer planet, which is known as retrograde motion. Planetary motion takes a lot of patience to fully appreciate. Mars takes over two years to complete a circuit of the sky, while Jupiter takes 12 years and Saturn takes 30.
The big question is: how do we tell a planet from a star without the wait? Stars appear as small points of light which means, when the light passes through our atmosphere, the star appears to 'twinkle'. In astronomical terms, the planets in our solar system are incredibly close; about 10,000 times closer than the next star. For this reason, although it may not be obvious, planets appear as discs rather than points of light and therefore the atmospheric effect is less noticeable.
As a result, you will see something that looks like a star but if you concentrate, you will notice it does not twinkle. Some of the planets are also very bright. Venus is the closest planet to Earth and has a highly reflective atmosphere. For these reasons, it is the brightest planet and is always the first 'star' you see at night or the last 'star' you see in the morning.
This month, Venus appears as the morning star and will rise a few hours before sunrise. Mars will rise at around midnight as a bright red 'star'.
Only Jupiter and Saturn will be visible in the evening sky. They are both currently at opposition which means they are visible all night long. Currently they are very close together so you will see a bright pale orange 'star' followed by a fainter yellow 'star'.
In December, Jupiter will pass in front of Saturn, so while you can see a gap between the planets now, it will close in a matter of months.
This 'conjunction' between the two planets happens once every 20 years.
