Twilight vs. Light Pollution

A few weeks ago I was waiting for it to get dark so I could go out into my garden and use my telescope. I decided to wait until the end of astronomical twilight (when there is no light from the Sun left in the sky) so that the sky was “properly dark”, but of course given that I live in a city (Glasgow) the light pollution from street lights means that it never actually gets “properly dark”.

So I decided to figure out exactly how long I should wait after sunset before going out to observe, or put another way, at what point does light pollution take over from twilight as the dominant source of light in the sky?

Sunset behind Glasgow Science Centre

I carried out this little experiment on 28 April 2011, in my back garden on the Southside of Glasgow, under the following conditions:

Sunset Time: 2050 BST
Civil Twilight Ends (Sun 6 degrees below horizon): 2134 BST
Nautical Twilight Ends (Sun 12 degrees below horizon): 2232 BST
Astronomical Twilight Ends (Sun 18 degrees below horizon): 2358 BST
Longitude: 55.866 N
Latitude: 4.257 W
Sky conditions: 100% clear

[Incidentally, sitting out in my garden for four hours as it darkened was an absolute delight: I saw, as well as the emerging stars, many bats, some ducks, two foxes, and two passes of the International Space Station!]

Using a Sky Quality Meter I took readings of the sky brightness at the zenith every two minutes. The SQM-L makes measurements of the sky in magnitudes per square arcsecond, i.e. brightness per unit area in the sky. As a general rule, in city centres you would expect readings of 16-17, while in dark places you can get readings of 21-22. The higher the number the darker the sky. The darkest reading possible under a starry sky is around 22, as at that point the starlight itself becomes the limiting factor.

From experience I know that in my garden the darkest reading possible is around 18.5, so I decided to continue taking readings until I got fifteen in a row that were above 18.4, i.e. for half an hour the sky had not been significantly darkening. I got my first reading of above 18.4 at 2244 and the sky did not appreciably darken over the next 30 minutes.

At 2244 the sun was 13 degrees 07 minutes (13.117 degrees) below the horizon.

I graphed the results to see how they looked, and placed them alongside the projected results if I were under a dark sky free of light pollution i.e. so that the results could get as low as 22.0 at the end of astronomical twilight at 2358 BST

SQM-L Readings vs Time

Result: After sunset, and throughout civil twilight (Sun between 0 and 6 degrees below the horizon) and nautical twilight (Sun between 6 and 12 degrees) the dominant source of light in the sky is sunlight.

However shortly after the end of nautical twilight light pollution became the dominant source of light in the sky, when the Sun was a little more than 13 degrees below the horizon.

This means that, while observing from my garden in Glasgow, I shouldn’t wait much later than the end of nautical twilight to go out observing, since beyond that point the sky will not significantly darken.

CAVEAT: It should be noted that this information is really only relevant for my specific circumstances, i.e. the light pollution in your sky may be better of worse than mine, and mean that the point at which it begins to dominate twilight is different for you.

Equinox, Equilux, and Twilight Times

On or around 21 March each year it is the Spring Equinox in the Northern Hemisphere, the day when eggs can be stood on their ends, when the Sun crosses the celestial equator, and when night and day are equal length the world over.

But this last part isn’t entirely true, for a variety of reasons.

Equinox

Equinox is latin for aequus (equal) and nox (night) meaning, roughly, “equal night”. The moment of the equinox is defined as the point at which the centre of the Sun’s disk crosses an imaginary line in the sky called the celestial equator, the projection of the Earth’s equator out into space.

The Sun (and the Moon and all the planets) move along a line in the sky called the ecliptic, the projection of the disk of the solar system out into space. These two lines, the equator and the ecliptic, cirlce the sky, and because the Earth’s axis is tilted at 23.5 degrees the angle between the equator and the ecliptic is 23.5 degrees, and the two circles meet at only two points, called equinoctial points.

Over the course of the year the Sun, as seen from Earth, appears to make one complete circuit around the ecliptic, as the Earth in fact orbits the Sun. And so on two days each year the Sun’s path crosses the equator. This means a number of things:

1. that an observer at the equator will see the Sun directly overhead at mid-day on the equinoxes
2. that the Sun will rise due east and set due west on the equinoxes (on all other days the Sun will rise either north or south of east, and set north or south of west)
3. the length of day and night are nearly equal

On this last point, they are not exactly equal, for two reasons:

1. the Sun appears as a disk in the sky with a radius of around 16 arcminutes, and so the top of the Sun appears to rise while the centre of the disk is still below the horizon, and the instant of the equinox is measured with respect to the Sun’s centre, and
2. the Sun’s light is bent, or refracted, in the Earth’s atmosphere, so that rays from the Sun can light you up even before the Sun rises, and keep you lit after it sets, with the degree of refaction being around 34 arcminutes

These two factors combine to mean that the Sun will appear to have “risen” when the centre of the disk is still 50 arcminutes (16 + 34) below the horizon, making the amount of daylight longer than the expected 12 hours. How much longer depends on where on Earth you are, but in the UK the length of the day is approx. 12 hours 10 minutes, rather than exactly 12 hours.

Equilux

Because of this effect, the days on which the length of day and night are exactly equal, called the equilux, occur a few days before the spring equinox and a few days after the autumn equinox. This date will vary depending on where on Earth you are, and indeed equiluxes do not occur at all close to the equator, whereas the equinox is a fixed instant in time.

Twilight

But the story doesn’t end there. Even on the days of equilux, the sky will have been bright for some time before the first rays of the Sun hit you, and will remain bright for some time after the last rays disappear from view. This time of day is called twilight, starting at dawn and ending at dusk.

There are, in fact, three kinds of twilight.

Civil twilight is what most people mean when they talk about twilight. It starts in the morning when the centre of the Sun’s disk is 6 degrees below the horizon, and ends at sunrise. In the evening civil twilight stars at sunset and continues until the centre of the Sun’s disk is 6 degrees below the horizon. During civil twilights the sky is still bright enough that, in general, artificial illumination will not be needed when doing things outside. In reality though, most councils switch lights on a fixed time after sunset (say 30 minutes) and turn them off a fixed time before sunrise, rather than relying on the Sun’s angular distance below the horizon.

Nautical twilight is when the Sun’s disk is between 6 and 12 degrees below the horizon. During nautical twilight it is still possible to distinguish the sky from the distant horizon when at sea, thus allowing sailors to take measurements of bright stars against the horizon. Most of us would consider this “dark”, but it is still technicaly twilight.

Astronomical twilight is when the Sun’s disk is between 12 and 18 degrees below the horizon. During astromical twilight you’ll no longer be able to tell the sky from the distant horizon when at sea, but crucially for astronomers, there is still light in the sky. Not much – indeed most of us would say it’s properly night time at this point – but the faintest objects in the sky, such as nebulae and very dim stars, will only be visible after astronomical twilight ends. Also, if you intend to measure how dark the sky is using a Sky Quality Metre, say, it is important to wait till after astronomical twilight.

So how much daytime does this twilight add? Only during civil twilight can the sky really be considered “light”, and so we can say that the day begins (at the point we would call dawn) at the start of morning civil twilight, and ends, (at the point we would call dusk) at the end of evening civil twilight. This means that on the Spring Equinox, the “day” will be around 13 hours and 15 minutes long in the UK (depending on where you are).

The date at which the “day” (including dawn and dusk) is 12 hours long in the UK (and therefore when we had equal amounts of daytime and nighttime) occurs around 01 or 02 March, over two weeks before the so-called equinox!

To find out your local sunrise / sunset / twilight times visit www.timeanddate.com