Who doesn’t love summer? School’s out and it’s the time of year for camping trips and beach holidays! Plus, the long days mean there’s time for another ice cream as we watch the day melt into a sunset. Extra sprinkles, please.
Late sunsets mean night falls later, too. But, while the kids aren’t in school, us adults still have to get up for work.
In the UK, the July Sun doesn’t set until after 21:00 BST (20:00 UT), and the last drops of sunlight don’t leave the sky until midnight.
So there isn’t much true night when we’re this close to June’s summer solstice, but there’s plenty of sky-watching we can do, especially if we start earlier, during evening’s twilight.
So much can be seen as the Sun begins to set and the world gets a little darker. The twilight sky is largely too bright for deep-sky objects, so it’s best to focus on what we can see with the unaided eye.
If you do use binoculars or a scope to get a better glimpse at evening objects, remember: never look directly at the Sun, especially through magnification, unless you have special filters for solar viewing.
You might also find it helpful to block the Sun with a large object, like the side of a building. Now, let’s see what twilight has to offer…
Stargazing during twilight
While afternoon slowly turns into evening, we start the first stage of twilight. Civil twilight begins in London in the evening at 21:20 BST (20:20 UT) at the start of the month, and 20:49 BST (19:49 UT) at the end.
The effects of twilight aren’t only in the sky. This might sound strange, but now is a great time to look away from the sky.
With the Sun nearing the horizon, and the low atmosphere scattering more light, the pinks, oranges and purples of sunset reach out across the landscape.
All around us the shadows of trees and posts lengthen, and the colours of cars and houses take on a soft orange glow.
Don’t forget to listen as well. Are there new bird songs? Can you hear any foxes or other animals scurrying around? As darkness settles, the bees and butterflies head home, while moths and bats take to the sky.
When the Sun falls to about six degrees above and then drops below the horizon, the shadows dissolve away.
We’re in the Golden Hour, so-called because of the soft, golden colour the remaining sunlight gives to everything we can see. There are no shadows left and the colours around us make it a good time for photography.
After this comes the Blue Hour, when the last of the remaining sunlight scatters and casts a blue glow into the dusk. We’re using the word ‘hour’ loosely here. It refers to the part of the day, not to an actual amount of time.
This is when we might be able to see Earth’s shadow – the same shadow that brings us a lunar eclipse – cast onto the atmosphere as a broad and dark band just above the horizon opposite the Sun.
Above it is the Belt of Venus, which is caused by scattered sunlight a bit higher from the ground, and this tends to be closer to pink.
What to see during twilight, July 2021
As afternoon ends and another July night settles in, check these celestial sights off your twilight observing list
Late afternoon: It’s easiest to find the Moon in the southwest a couple of days before and after first quarter (approx. 15-19 July).
21:30 BST (20:30 UT): Near sunset, you might spot Venus in the west. Vega (Alpha (α) Lyrae) in the east, Arcturus (Alpha (α) Boötis) in the south, Antares (Alpha (α) Scorpii) low in the south, and Capella (Alpha (α) Aurigae) in the northwest are the first stars to be visible.
For more targets to spot, read our pick of the best summer stars.
22:00 BST (21:00 UT): You’ll see second, third and fourth magnitude stars. It’s easier to see the stars of the Spring and Summer Triangles (one of our favourite of the summer constellations and asterisms) high in the east and south, and the Plough too. The constellation of Scorpius, the Scorpion is also an easy spot.
The first objects to appear during twilight, mid-July 2021
- Kaus Australis
- Polaris (the North Star)
Spotting the Moon during twilight
Depending on where it is in its orbit – which of the phases of the Moon we can see – the first thing we’ll see through the twilight is the Moon.
The Moon is new on 10 July 2021. In the days that follow, look for thin, sleepy crescents lagging behind the Sun.
These early phases are a great time to look for earthshine: sunlight that bounces off Earth onto the Moon.
It gives the Moon’s night-time side a gorgeous dim and dusty glow. Who knows, maybe some of the photons we’re seeing bounced off us!
As the week goes on, the Moon sets later and later, moving farther to the east (to our left) by about 13˚ each night.
On 17 July it reaches first quarter and doesn’t set until before midnight. For a couple of days on either side we can see it in broad daylight without too much trouble, in that strange part of the day that was evening a few months ago but is still afternoon now.
This is a great time to point a pair of binoculars or a small telescope at our nearest neighbour. There’s an understated, stark beauty to seeing it in the late afternoon, and the sight of long shadows on the Moon’s craters in the calm glow of early evening twilight has a soothing quality, even after the toughest of days.
Spotting the planets during twilight
The next things we’re likely to see in the deepening dusk are our Solar System’s bright planets. Though we don’t typically think of the planets as twinkling in the way stars do, we might notice it a bit in the early evening as their light struggles with the low-lying atmosphere.
It’s always fun to try to find planets in this part of the evening as they make their way through twilight.
The sky is still too bright for most stars, which makes it easier to be sure we’re looking at a planet. Once you find them, keep an eye on them until they set, and then follow them as they wander across the sky from evening to evening.
For more help, read our guide on how to find the planets.
Venus and Jupiter are the two brightest things in the sky after the Sun and the Moon, so depending on where they (and the Moon) are in the sky, they might be the first things we see.
Venus is stunning in a deep, dark sky, but the twilight version has a simple, understated beauty. It seems to arrive without any complications or celebrations.
Although its position is on the low side in July 2021, you’ll find it in the western twilight and it sets just after the Sun, where it almost feels like spotting a friend waving through a crowd.
Mars will be there this month too, but it’s small, and its reddish-orange colour blends in somewhat with the sky around it.
Through July, Venus rises a little towards the west, while Mars sinks towards the northwest each night.
Step out on the evening of 13 July and the two planets will be less than half a degree apart just above the western horizon, with a young crescent Moon nearby.
Binoculars will help separate the two planets in this stunning scene. Although Mars will vanish by early August, Venus stays visible but is low to the horizon.
Early twilight is a good time to try and find speedy Mercury. The innermost planet never gets more than 28˚ of arc from the Sun, so most of the time it’s only visible in twilight. This month, we might be able to spot it in the morning twilight, just before sunrise.
Jupiter and Saturn don’t rise until a bit deeper into the night this month. But, if you want to stay up a later, a regular pair of binoculars will reveal Jupiter’s four giant Galilean moons. And, thanks to its rings, Saturn might look a little egg-shaped.
Deep-sky objects and noctilucent clouds
Keep an eye on the stars you’ve found so far and make your way into astronomical twilight – 23:19 BST (22:19 UT) in early July, and 22:24 BST (21:24 UT) at the end.
By now, you can see Antares (Alpha (α) Scorpii), the red supergiant in the constellation of Scorpius, the Scorpion, which is low towards the south and has been hidden away by lingering sunlight.
Next, look for some of the other stars near the Summer Triangle. Can you see the Northern Cross? How about The Teapot asterism of Sagittarius, the Archer? You’ll find it chasing the Scorpion, low towards the southeast.
These stars are mostly of second, third and even fourth magnitude. The dimmer they are, the longer they’ll take to make their way into the night. Scan the skies with binoculars or using a small telescope.
During astronomical twilight in the summer, you might also be able to see noctilucent clouds. These aren’t true clouds, but ice crystals high in Earth’s atmosphere that reflect sunlight and
look like wispy, delicate clouds.
The further north you are, the better luck you’ll have seeing them. Just make sure you have a clear northern horizon and look while the sky is still just about lit by the now-set Sun.
Point binoculars towards the noctilucent clouds and see what their delicate and wispy patterns look like magnified.
After this, the sky keeps darkening until our part of the world has turned fully away from the Sun. It’s night at last, and perhaps time to head back inside until tomorrow’s twilight.
As wonderful as they are, summer nights can be short and difficult for those with a hankering for true darkness. But with a little bit of patience and luck, and some good timing, there are some truly incredible things to look for before dark.
What is twilight?
Twilight is not a single, fixed state, but a gradual change that has three distinct phases. The changes that occur during dusk can be as striking as anything we observe in nature. Everything we can see changes, as the brightness of the sky drops to less than 3/10,000ths of a per cent of its intensity at sunset.
Yet this daily spectacle is often lost to us, perhaps obscured by cloud, but also obliterated by artificial lighting and sometimes simply ignored because of its regularity.
The stages of twilight
During civil twilight the Sun has set, but there is still enough light left to see without streetlights. This stage lasts until the Sun’s centre is 6° below the horizon.
When the Sun is between 6° and 12° below the horizon it is nautical twilight. Extra light is needed
to help walk around, but enough celestial objects become visible to navigate by the stars.
The last stage of evening twilight is called astronomical twilight. It’s when the Sun’s centre is between 12° and 18° below the horizon. There is still some lingering sunlight, but it’s dark enough for astronomy work to begin. After this, night begins.
This is their order in the evening. Before dawn, they’re reversed: night, astronomical, nautical and then civil twilight.
What can you see during twilight?
Words: Steve Tonkin
We tend to look to the west at sunset, drawn by the coral pink hues above the horizon, and miss the more dramatic changes that are happening behind us.
Here, we see a band of more muted amaranth pink, dubbed the Belt of Venus, illuminated by red sunlight that is not scattered in its passage through the atmosphere. Below is a rising purple swathe, that part of the visible sky that is in Earth’s shadow.
During civil twilight, only the very brightest stars and planets become visible.
Civil dusk signals the beginning of nautical twilight, which persists until the geometric centre of the Sun is 12° below the horizon – nautical dusk.
At nautical dusk, it’s sufficiently dark that a sailor at sea would not be able to see the horizon, hence its name.
Our monochrome scotopic (low light) vision begins to dominate and colours fade as everything on land takes on shades of grey. The purple in the east merges with darkening sky above. First-magnitude stars begin to appear.
Initially they seem lonely points of light, but they gradually multiply as the sky darkens and fainter stars join them.
Eventually, the entire Plough asterism in Ursa Major appears, pointing to Polaris, so at last we can polar align our equatorial mounts.
Night is approaching, but the sunlit sky is still visible on the sunset horizon. The third phase, astronomical twilight, is beginning.
What is astronomical twilight?
As the Sun descends past nautical dusk and into astronomical twilight, when our star is between 12° and 18° below the horizon, its illumination is replaced by other sources.
For too many of us, this is the skyglow from artificial light, but even in unlit places on a Moonless night the sky is never completely dark.
The combination of an imperceptibly faint auroral glow, the zodiacal light (sunlight reflected off interplanetary dust particles), and the light of diffuse matter in our Galaxy all contribute, though their contribution is less than that of a single mag. +5.6 star if it was distributed over an area the size of the Moon.
Astronomical dusk takes place when the Sun’s geometric centre drops to 18° below the horizon. Above our heads we will see, with dark-adapted eyes, objects as faint as we are likely to.
Away from light pollution, the Milky Way shows structure sculpted by the dust of dark nebulae. The Andromeda Galaxy and the Double Cluster in Perseus may show themselves even without binoculars.
The varied colours of stars become more apparent, and our awareness of the existence of artificial satellites and sporadic meteors grows.
The glittering sky-dome above our heads appears to have come closer. This is night. Then, all too soon, it is over. The sky brightens, the stars fade, the twilight phases play out in reverse.
Dawn, and a brand-new day, is upon us.
Why is twilight shorter in winter than in summer?
You can visualise the reasons by considering the sky as a dome, with the Sun appearing to trace a curved path (known as the ecliptic) across it each day.
In winter, when the Sun’s path lies in the south of the dome, the Sun’s path after sunset curves down away from the horizon.
The effect is to gradually increase the angle at which the Sun appears to travel with respect to the horizon, decreasing the duration of twilight.
In the summer, when the Sun’s path is in the north of the dome, the path curves up to the horizon.
This has the effect of gradually decreasing the angle the Sun appears to travel with respect to the horizon, lengthening twilight.
For more on this, read our guide How does Earth orbit the Sun?
Why is twilight shorter closer to the equator?
This is best described at the equinoxes, when the Sun sets at a right angle to the horizon at the equator and it has to sink 18° after sunset for night to begin.
Earth rotates at 15° per hour, so twilight at the equator will last 72 minutes (18°/15°×60 minutes).
The angle of sunset is equal to 90° minus your latitude, so at 51°N the Sun sets at an angle of 39° relative to the horizon.
Using trigonometry you can work out the Sun has to travel for 28.6° before it is 18° below the horizon.
Twilight at 51°N will therefore last for 114.4 minutes (28.6°/15°×60 minutes).
Scott Levine is a naked-eye astronomer and astronomy writer based in New York’s Hudson Valley. for more of Scott’s stargazing tips, visit his website Scott’s Sky Watch.
Stephen Tonkin writes BBC Sky at Night Magazine’s Binocular Tour each month.