Under the current definition of a planet, Neptune is the farthest such object in our Solar System. It orbits the Sun at a distance of 4.5 billion kilometres, taking 164.8 years to complete each orbit.

Neptune was discovered by Johann Galle and Urban Le Verrier on 23 September 1846 and as such has only completed one orbit around the Sun since its discovery.

Neptune is the only main planet not visible to the naked eye, although its Solar System neighbour Uranus is hardly easy in this respect!

Find out what to observe in the night sky every month in our Star Diary podcast, or prepare for the dark months ahead with our guide to autumn astronomy.

In September 2021, Neptune is located in Aquarius, roughly midway between mag. +4.2 Psi (φ) Aquarii and mag. +5.5 20 Piscium.

Binoculars will show it to look exactly like a mag. +7.8 star, but its planetary nature becomes more evident through the eyepiece of a telescope.

Neptune shows a tiny 2 arcsecond disc. A power of 200x or greater is recommended to show this disc well.

The planet’s colour, like the green hue of Uranus, is quite striking, Neptune being noticeably blue.

Neptune reaches opposition on 14 September although being so distant, this optimal position in the sky doesn’t make a great deal of difference to the planet’s overall appearance.

It does mean it’s up all night long though, so this is as good a time as ever to look for it.

Smaller instruments will show the planet’s blue disc as described, but may also be able to pick out Neptune’s largest moon, Triton.

Shining at mag. +13.5, Triton is an easy catch for a 300mm instrument, but isn’t out of range of smaller scopes; 200mm is probably the minimum.

Larger scopes fitted with specialist high-resolution imaging kit may occasionally pick out large detail on Neptune’s disc such as atmospheric banding and storm systems.

How to see the planets in September 2021

Neptune

• Best time to see: 14 September, 01:00 BST (00:00 UT)
• Altitude: 33º
• Location: Aquarius
• Direction: South
• Features: Small blue-hued disc, atmospheric features
• Recommended equipment: 200mm or larger

Mercury

• Best time to see: 1 September, 15 minutes after sunset
• Altitude: 2˚ (very low)
• Location: Virgo
• Direction: West

Mercury is an evening object during September 2021, but its position deteriorates over the month. Your best chance of spotting it will be on 1 September when, shining at mag. 0.0, it sets 30 minutes after the Sun, below the western horizon.

This doesn’t give you long to locate it. Venus is also low in this direction, 16˚ east of Mercury’s position. By the end of the month, mag. +1.5 Mercury virtually sets with the Sun.

Venus

• Best time to see: 1 September, from 20 minutes after sunset
• Altitude: 5˚ (low)
• Location: Virgo
• Direction: West-southwest

Venus is visible in the evening sky after sunset, appearing higher than Mercury and much brighter. At the start of September Venus shines at mag. –4.0. By the end of the month it increases in brilliance to mag. –4.2. Venus sets approximately 1 hour after the Sun throughout the month.

Given a flat west-southwest horizon, it may be possible to spot Spica (Alpha (α) Virginis) 1.6˚ below Venus on the evening of 5 September.

Mars

Shining with a magnitude of +1.8, Mars isn’t really a viable target this month as it is too close to the Sun to be seen.

Jupiter

• Best time to see: 1 September, 00:20 BST (23:20 UT)
• Altitude: 22.9˚
• Location: Capricornus
• Direction: South

Jupiter is an evening planet, managing to achieve its highest position in the sky due south in darkness during the month. It’s apparent position in the sky has it travelling west through the eastern part of Capricornus, the Sea Goat and this will reduce its overall altitude over the month.

On 1 September, from the centre of the UK, Jupiter attains an altitude of 22.9˚. By 30 September, this value will have dropped to 22.0˚.

Saturn

• Best time to see: 1 September, 23:10 BST (22:10 UT)
• Altitude: 18˚
• Location: Capricornus
• Direction: South

Being at opposition at the start of August and with the rapidly expanding nights that occur during September, Saturn remains nicely placed, able to attain its maximum altitude of around 18˚ from the centre of the UK under dark sky conditions all month long.

On 1 September, mag. +0.3 Saturn reaches its highest point in the sky, due south, at 23:10 BST (22:10 UT). By the month’s end, this position is reached at 21:10 BST (20:10 UT).

Uranus

• Best time to see: 30 September, 03:15 BST (02:15 UT)
• Altitude: 53˚
• Location: Aries
• Direction: South

Uranus is well placed, reaching a position of 50˚ in total darkness in the morning sky. It currently resides in Aries, the Ram and is not too far from the Pleiades open cluster, the cluster being 16˚ east-northeast of Uranus.

By the end of September, Uranus reaches a maximum height of 52˚ in darkness when due south as seen from the centre of the UK.

This guide originally appeared in the September 2021 issue of BBC Sky at Night Magazine.