What planets will be in the night sky this month, and when best to observe them.
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The planet Saturn reaches opposition on 20 July 2020. ‘Opposition’ is the term used to describe when a planet lies on the opposite side of the sky to the Sun. In this position we are at the closest to that planet and consequently it appears bigger and brighter than at other times.
The more distant a planet is from the Sun and therefore Earth, the smaller these effects become.
For example, we’re going to see major changes in the appearance of Mars as we head through to its opposition on 13 October.
Jupiter too will be at its brightest and largest through the eyepiece on 14 July, but the changes are less impressive than those which will appear for Mars.
And the progression continues through to Saturn with its brightness and increase in apparent size being fractionally less impressive than that which occurs with Jupiter.
Saturn’s rings can appear to brighten significantly at opposition. Credit: Pete Lawrence
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Having said this, Saturn still has a trick up its sleeve in the form of the Seeliger effect. As the planet approaches opposition, the myriad particles which make up its ring system line up so that from Earth the shadows they cast on particles further back are hidden from view.
The net effect is a brightening of the rings. This effect can normally be seen a few days before opposition, reaching a peak brightness at opposition and then fading off in the days after.
Saturn and Jupiter reaching opposition within a week of each other occurs as a consequence of both appearing close in the sky. On 20 July Saturn appears 7.1° east of Jupiter.
At opposition, Saturn’s brightness will be mag. +0.4. A full Moon – the Moon at opposition – lies near to both planets on the evening of 5 July and into the following morning.
The phase and relative sizes of the planets, July 2020. Each planet is shown with south at the top, to show its orientation through a telescope. Credit: Pete Lawrence
How to see the planets this month
Saturn
- Best time to see 20 July, From 00:00 BST (23:00 UT)
- Altitude 16°
- Location Sagittarius
- Direction South
- Features Rings, atmospheric belts, occasional storms, moons
- Recommended equipment 150mm or larger
Mercury
- Best time to see 31 July, 1 hour before sunrise
- Altitude 4° (very low)
- Location Gemini
- Direction Northeast
Mercury reaches inferior conjunction on 1 July and then returns to morning skies. It’s poorly placed in July’s first half but improves from the 16th, as it brightens and appears higher before sunrise. On 16 July it rises over the northeast horizon an hour before sunrise. Greatest western elongation (20.1°) occurs on the 22nd.
Venus
- Best time to see 31 July, 04:00 BST (03:00 UT)
- Altitude 14°
- Location Taurus
- Direction East-northeast
Morning planet Venus shines at mag. –4.3 at July’s start, with a scope showing it as an 18%-lit crescent, 42 arcseconds across. On 12 July, mag. –4.4 Venus appears less than a degree from mag. +0.8 Aldebaran.
Venus is 28%-lit and 36 arcseconds across on this date. A waning crescent Moon sits 2.6° from Venus on 17 July, both visible close to the Hyades at around 03:00 BST (02:00 UT). At the month’s end, Venus rises three hours before sunrise, shining at mag. –4.3.
Mars
- Best time to see 31 July, 04:00 BST (03:00 UT)
- Altitude 35°
- Location Pisces
- Direction Southeast
Mars improves this month. On 1 July it hovers low above the east-southeast horizon as the sky starts to brighten. Visually it shines at mag. –0.5 and through the eyepiece its apparent size is 11 arcseconds.
As Mars rises around 1am on 12 July, it shines at mag. –0.7 and appears 3° from a waning gibbous Moon. On this date Mars attains a higher altitude in darker skies, being over 20° up by 03:00 BST (02:00 UT).
At July’s end, Mars shines at mag. –1.1 and will look impressive as it reaches an altitude of 35°. On 31 July, Mars has a 14 arcsecond disc 84%-lit when viewed through a scope.
Jupiter
- Best time to see 14 July, 01:00 BST (00:00 UT)
- Altitude 15°
- Location Sagittarius
- Direction South
Jupiter reaches opposition on 14 July, appearing at its brightest and largest in 2020. At mag. –2.6 it will be impressive visually, but it’s low and this will reduce the detail through a scope. A telescope view will still show the main atmospheric belts and four largest moons. The full Moon on 5/6 July lies close to Jupiter and Saturn.
Uranus
- Best time to see 31 July, 02:30 BST (01:30 UT)
- Altitude 23°
- Location Aries
- Direction East
A morning planet shining at mag. +5.8, Uranus is able to reach an altitude of 30° in darkness at July’s end.
Neptune
- Best time to see 31 July, 02:30 BST (01:30 UT)
- Altitude 29°
- Location Aquarius
- Direction South-southeast
Morning planet Neptune almost makes it to its highest altitude due south at the month’s end. Currently in Aquarius, at mag. +7.8, it requires binoculars to see.
Pete Lawrence is an experienced astronomer and a co-host of The Sky at Night. This guide originally appeared in the July 2o20 issue of BBC Sky at Night Maagzine.
