If you happened to gaze upward last night, you might have noticed a very bright point of light next to the Moon. And for the eagle-eyed among you, you’ll have noticed that, unlike stars, this point of light is not twinkling.
This is because it’s a planet. The granddaddy of the Solar System in fact; the whirling giant that is Jupiter. It can be seen for around 10 months of the year, but as it approaches opposition in early November, it’s becoming a prominent figure in the night sky.
Found within the zodiac band of the sky (currently in Aries along with Uranus), Jupiter travels from west to east, but as of yesterday, 4 September 2023, has entered into a period of retrograde motion, which will last until New Year’s eve. Essentially, this means that Jupiter has started to move backwards, from our point of view on Earth.
And it’s a great planet to watch; Jupiter is always brighter than the binary star Sirius, which is the brightest in the night sky.
The brightness of objects in the sky is measured in orders of magnitude. It uses a logarithmic scale, with lower numerical values representing brighter objects and higher values indicating fainter ones. In other words, the lower the magnitude, the brighter the object, with minus figures indicating a really bright object.
Jupiter is currently at magnitude -2.48 and is gradually getting brighter as we head into the autumn, reaching -2.76 by 27 October, before reaching opposition on 2-3 November.
How can I see Jupiter tonight?
Tonight, 5 September 2023, the planet Jupiter will be visible next to the Moon. It won’t be quite as close to the Moon as it was last night, but with clear skies forecast it will still be an impressive sight.
Jupiter is easy to find; look towards the north-eastern horizon from around 10pm onwards, where you’ll be able to spot it, low on the horizon to the east of the Moon.
Tonight, 5 September, the waning gibbous Moon rises at 9:43pm into the northeast and will be at around 68 per cent illumination.
Looking up at around 11pm will offer good views of the pair, once they’ve both risen above any interfering vegetation on the horizon.
Jupiter’s Galilean Moons
Did you know that you can see Jupiter’s four largest Moons with just a pair of binoculars?
The largest of the four, Ganymede, is bigger than Mercury, and nearly half the diameter of the Earth. All four of the Galilean moons, Io, Europa, Ganymede and Callisto, are larger than our own Moon, and along with Saturn’s moon Titan, are some of the largest moons in the Solar System.
All four Moons are currently nicely spaced out in a diagonal, so with a pair of binoculars and a steady pair of hands (or a tripod) you can easily distinguish between them.
From our perspective, they are in the following order, from lower left to upper right:
Callisto – Ganymede – Io – JUPITER – Europa.
Why do stars twinkle, and planets don’t?
It all comes down to our old friend; the reason the sky is blue and why the Moon sometimes looks red: the atmosphere. Because stars are located so far away, even through a telescope they appear as a single point of light; you can’t make out any detail. But when you look up at a star, it twinkles.
This is down to turbulence in the atmosphere, refracting the starlight in different directions. This makes the star appear to change brightness and position, which to us here on Earth, makes it look as if it’s twinkling.
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