How to see the planets in the night sky, July 2021

How to see the planets in the night sky, July 2021. Credit: m-gucci / Getty Images.

Jupiter rises five hours before the Sun on 1 July 2021 and manages to reach a peak altitude of 25˚ at 04:50 BST (03:50 UT) in a bright pre-sunrise dawn sky.

Although this may appear low, it’s a considerable improvement over the 14 or so degrees Jupiter was able to achieve in 2020.

Indeed, an increase in altitude of 10˚ can make a huge difference to the appearance of the planet through a telescope.

In July 2021, Jupiter’s position in the sky will be higher than it was throughout 2020. Credit: Pete Lawrence.
In July 2021, Jupiter’s position in the sky will be higher than it was throughout 2020. Credit: Pete Lawrence.

Two of the main effects that hinder viewing are reduced. The first is the result of the thicker layer of atmosphere between us and the planet when it’s low down.

As the planet rises higher, the thickness reduces and this translates directly to a reduction in the instability of the view.

When viewed through a telescope, Jupiter looks more stable when it’s close to its highest position in the sky, due south.

Another effect that impacts low altitude objects comes from the atmosphere’s ability to disperse light.

Much like the way white light is spread into its component colours through a prism, the same happens with our atmosphere.

Jupiter Rouzbeh Bidshahri, Dubai, 25 July 2019. Equipment: ZWO ASI 290 mono camera, Celestron C14 Schmidt-Cassegrain telescope, Losmandy Titan mount.
.Jupiter is approaching opposition, appearing at its best through a telescope on 31 July. Credit: Rouzbeh Bidshahri, Dubai, 25 July 2019.

The thicker the atmospheric layer the light has to pass through, the greater the dispersion.

This manifests as colour fringing around a bright planet’s edge; with red at the bottom and blue/green at the top. Jupiter’s altitude will also reduce this.

Jupiter is close to equinox and there are a number of mutual events visible between its moons as a result.

A 94%-lit waning gibbous Moon sits 5˚ south of Jupiter on 26 July, when both objects are due south at 03:00 BST (02:00 UT).

On 31 July, Jupiter reaches its highest position in the sky, due south, at 02:40 BST (01:40 UT) just after true darkness ends.

The phase and relative sizes of the planets in July 2021. Each planet is shown with south at the top, to show its orientation through a telescope. Credit: Pete Lawrence
The phase and relative sizes of the planets in July 2021. Each planet is shown with south at the top, to show its orientation through a telescope. Credit: Pete Lawrence

How to see the planets in July

Jupiter

Mercury

Mercury is a morning object, rising about an hour before the Sun on 1 July. It ascends in the northeast, shining at mag. +1.0 and showing an 8 arcsecond disc that is 27% lit.

On 8 July a 3%-lit waning crescent Moon lies 2.9˚ to the north of the now mag. +0.2 planet. Mercury rises 80 minutes before the Sun from 8–15 July.

Over the rest of the month, it heads nearer the Sun, rising 25 minutes before sunrise on 31 July.

It also brightens and this should help to keep it visible. On 15 July it shines at mag. –0.6, increasing to mag. –1.6 on 27 July when it rises 45 minutes before the Sun.

Venus

Venus is an evening planet, suffering due to poor placement. Shining at mag. –3.9, it sets 1.5 hours after the Sun on 1 July, a figure reducing to 70 minutes after the Sun on 31 July.

It lies 30 arcminutes from mag. +1.9 Mars on 13 July.

Mars

Mars appears tiny through the eyepiece, less than 4 arcseconds across. It is struggling to keep ahead of the evening twilight and is barely visible in the west-northwest after sunset.

At mag. +1.8, Mars is outshone by Venus, which passes half-a-degree to its north on 13 July.

Saturn

Saturn is a morning planet, which is able to reach its highest position above the southern horizon in relative darkness in July.

Opposition occurs on 2 August and the end of July is a time when Saturn’s rings should be starting to increase in brightness due to the opposition effect.

A full Moon sits to the southeast of Saturn on the night of 24/25 July.

Saturn increases in brightness by a small amount: on 1 July it shines with an off-white, straw-yellow hue
at mag. +0.4 and by July’s close it increases to mag. +0.2.

Uranus

Uranus is slowly crawling away from the Sun but is not well positioned at the moment. It does manage to reach an altitude of 22˚ above the eastern horizon under dark-sky conditions by the month’s end.

Neptune

Neptune may be seen under dark-sky conditions towards the month’s end, although it isn’t able to achieve its highest altitude due south. Neptune is in Aquarius, close to the border with Pisces; find it south of the Circlet asterism in Pisces.

Pete Lawrence is an experienced astronomer and a presenter on The Sky at Night. This article originally appeared in the July 2021 issue of BBC Sky at Night Magazine.