Night skies threatened by growing satellite interference

Alan Pickup

Author: Alan Pickup
The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 31st. An arrow shows the motion of Mars.

Edinburgh enjoys five hours of effective darkness in the middle of the night as May begins, but this dwindles to nothing by the month’s end as the Sun climbs 7° northwards and persistent twilight begins to swamp all bar the brighter stars. On the plus side, all five naked-eye planets are in view, with four of them emerging from the Sun’s glare at present, including the normally-elusive Mercury as it puts on its best evening show of the year.

Spend any time under the stars at this time of year, though, and you are likely to spot one or more earth satellites. Taking just a few minutes to cross the sky, they appear like moving points of light against the stars as they shine by reflecting sunlight. Many are steady in brightness but some, perhaps because they are tumbling or spinning, appear to pulsate or flash. They may also disappear or reappear as they enter or exit the Earth’s shadow. That shadow stretches high overhead during most of our winter nights when, except for a few hours around dusk and dawn, few satellites are visible.

For the next few months, though, the shadow is shallow and most satellites passing overhead can still see the Sun even during our midnight hours. The problem for astronomers is that their light and radio emissions can interfere with observations, such as attempts to study the distant universe or scan for potentially hazardous asteroids.

The issue is becoming critical with the launch of multiple satellites such as those forming the Starlink megaconstellation, intended to provide global internet access from an altitude of 550 km. Elon Musk’s SpaceX company has approval to launch 12,000 Starlink satellites of which 1,354 were in orbit as of a few days ago.

While the Starlink design has changed to try to limit their brightness, all of them can be visible to the unaided eye. Indeed, a dozen or more Starlinks are likely to be making visible passes across our sky at any one time tonight, outnumbering the other non-Starlink craft. Other satellite megaconstellations are also under construction or planned by nations and companies such as China, Amazon and OneWeb, the latter now part-owned by the UK Government.

Andy Lawrence, the Regius Professor of Astronomy at the University of Edinburgh and Honorary President of our Society, has written a superb non-technical book, “Losing The Sky”, about the burgeoning threat from satellite megaconstellations, why it matters, and what we can do about it.

Mercury, the smallest and innermost planet, stands low in the west-north-west at nightfall, but we need the clear horizon at catch it as it climbs above the more brilliant evening star, Venus. Thirty minutes after sunset on the 1st, Venus shines at magnitude -3.9 but is less than 3° high and best seen through binoculars. Mercury is 5° above-left of Venus and, though it is fainter at magnitude -1.1, is visible for a further hour and is easier in binoculars as the twilight fades.

Mercury should be better still over the following evenings, becoming a naked-eye object as it stands higher and approaches its greatest angular distance of 22° from the Sun on the 17th. By then, Venus is 5° high thirty minutes after sunset, and Mercury is 8° above and to its left but has dimmed to magnitude 0.5. As Venus continues to climb higher, Mercury then falls back towards it, fading sharply to magnitude 2.2 by the time it lies 0.5° (one Moon-breadth) left of Venus on the 28th.

Sunrise/sunset times for Edinburgh change from 05:29/20:52 BST on the 1st to 04:36/21:46 on the 31st. The Moon is at last quarter on the 3rd, new on the 11th, first quarter on the 19th and full on the 26th when it slides though the northernmost part of the Earth’s shadow in a total lunar eclipse to be seen around the Pacific Ocean but not at all from Europe.

The slender young Moon is spectacularly earthlit as it reappears low in the west following new moon. It is barely 4% sunlit when it stands 2.7° below Mercury on the 13th and is 14% lit when it lies 4° below-right of Mars two days later.

Mars is the sole planet visible at our map times though we need to look increasingly low to find it in the constellation Gemini whose main stars, Castor and Pollux, stand side by side and almost due west as the evening twilight fades. Mars tracks to the left below them during May, falling between them in brightness as it dims slightly from magnitude 1.6 to 1.7.

As it encounters Mars on the evening of the 15th, watch for the Moon occulting the third magnitude star Mebsuta, or Epsilon Geminorum. Viewed through a telescope from Edinburgh, the star disappears for 21 minutes after it dips behind the Moon’s northern edge at 23:36 BST.

Our star charts show the Plough tumbling westwards from the zenith as Leo with its leading star Regulus sink in west. Look for the first quarter Moon 4° above Regulus on the 19th and midway between Regulus and the famous double star Algieba. The Moon moves on to lie 6° above-left of Spica in Virgo on the 23rd and 5° above-left of Antares in Scorpius on the 26th/26th.

For the final two bright planets we must look to the south-eastern horizon before dawn. This is where Saturn rises at 03:25 on the 1st and two hours earlier by the 31st. Shining at magnitude 0.6, it leads the impressively brighter Jupiter, magnitude -2.3, by some 30 minutes. The Moon lies 7° below-left of Saturn on the 4th and is 5° below the planet before dawn on the 31st.

Around 10° north-east of Jupiter is the radiant point of the Eta-Aquarids meteor shower whose peak is expected on the morning of the 6th. Its swift meteors, actually debris from Halley’s Comet, are much better seen from the southern hemisphere.

Diary for 2021 May

Times are BST

  • 3rd 18h Moon 4° S of Saturn
  • 3rd 21h Last quarter
  • 4th 00h Mercury 2.3° S of Pleiades
  • 4th 22h Moon 5° S of Jupiter
  • 6th 04h Peak of Eta-Aquarids meteor shower
  • 8th 12h Venus 4° S of Pleiades
  • 11th 20h New moon
  • 12th 23h Moon 0.7° S of Venus
  • 13th 13h Moon 2.1° S of Mercury
  • 16th 06h Moon 1,5° N of Mars
  • 17th 02h Moon 3° S of Pollux
  • 17th 07h Mercury furthest E of Sun (22°)
  • 18th 00h Venus 6° N of Aldebaran
  • 18th 03h Moon 3° N of Praesepe
  • 19th 19h Moon 5° N of Regulus
  • 19th 20h First quarter
  • 23rd 15h Moon 7° N of Spica
  • 23rd 21h Saturn stationary (motion reverses from E to W)
  • 26th 12h Full moon and total lunar eclipse
  • 26th 18h Moon 5° N of Antares
  • 29th 07h Mercury 0.4° S of Venus
  • 31st 02h Moon 4° S of Saturn

This is an extended version, with added diary, of Alan’s article published in The Scotsman on April 30 2021, with thanks to the newspaper for permission to republish here.