An astronomer's guide to twilight

The period between ‘day’ and ‘night’ is complex, and so is the sky at this time.

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Twilight is split into different grades, but the cause is the same: refraction and scattering

 

Written by Steve Tonkin

 

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.

Twilight is not a single, fixed state, but a gradual change that has three distinct phases.

The first is civil twilight, which begins as the upper limb of the Sun disappears below the horizon and ends at civil dusk, when the geometric centre of the Sun is 6° below the horizon.

During this period, you can carry on doing things much the same as if the Sun were above the horizon, lit only by the still-blue overhead sky.

The first half an hour being dubbed by photographers as the ‘blue hour’. 

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.

 


Nautical twilight: colours begin to fade and the horizon begins to disappear. More stars emerge.
Credit: iStock

 

Light’s last gasp

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. +6.5 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 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.

 

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).

 

Stephen Tonkin writes BBC Sky at Night Magazine's Binocular Tour each month.

 


 

 

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