Skywatcher’s Guide

Day time

Cumulus (L. heap)
Your basic fat, puffy clouds. Range from flocks of little Fair-Weather Cumulus up to vast
banks of cauliflowerish, sometimes-rainy Cumulus Congestus.

Cirrus (L. curl)
High, wispy ice-crystal clouds which come in all kinds of funky shapes.
Produce snow, but it usually evaporates before you notice it.

Stratus (L. layer)
A low layer of cloud, sometimes patchy with hazy edges.
A fog is a stratus right next to the ground.

Altostratus
Like a stratus, only higher up and usually deeper. Usually the sun is hazily visible through it.
Often precedes rain or snow.

Cirrostratus
Even higher up: a widespread, frozen, veil-like layer often producing a halo.

Cirrocumulus
A high-floating layer with lots of little fluffy blobs. Shows air is unsettled; rain likely later.

Altocumulus
A layer of cloud-rafts separated by thinner areas, pretty high up.
Can look a bit like ice floes breaking up in the sky, only much fluffier.

Stratocumulus
A clumpy layer in the middle sky, sometimes mixed with cumulus clouds.

Cumulonimbus
Big rain-clouds producing heavy, localised rain; usually flattened at the top
(miles up, where they freeze) and at the bottom (where they form).

Nimbostratus
Huge, country-engulfing rainclouds: Thick and dark with steady rain and hazy edges.
Thick and dark and wide with steady rain and hazy edges.

Optical Phenomena

Rainbows
Rainbows are caused by light bouncing once off the insides of raindrops; the colours occur because different colours (wavelengths) of light are refracted by different amounts.

Double Rainbows
Many rainbows have a double on the outside, the opposite way round from the main bow.
This is caused by light bouncing around the inside of water droplets twice. A third bow,
caused by light bouncing three times, is easily seen in the lab but almost never in the sky.

Supernumerary Rainbows
Occasionally a rainbow has many extra bands of colour close together on the inside. These
supernumerary bows show the interference pattern created by light waves leaving droplets;
their visibility depends on how uniformly sized the drops are.

22º Halo
A coloured ring around the sun, caused by refraction from cirrostratus.

46º Halo
Like the 22º Halo only bigger, weaker and rarer.

Sun Dogs
Bright, colourful patches of sky at the same elevation as the sun, just outside the 22º halo;
the result of refraction by ice crystals in cirrus clouds.

Circumzenith Arc
One of the more uncommon effects, this is a rainbow-arc partway around the zenith of the
sky (its highest point). Also produced by ice crystals.

Corona
A coloured ring pretty close to the sun or moon, caused by diffraction from water droplets in an altostratus or altocumulus layer. Droplets of different sizes give different-sized coronas, so the effect is smeared out if the sizes of the droplets vary too much. Although beautiful and common, these are usually much too bright to look at, which is why most people have never even noticed them.

Do not try to look for bright colours in the clouds immediately surrounding the sun without proper sunglasses! Even with strong shades, take great care not to look for too long at a time, and block the sun out of your view.

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Night time

The North Star
The side of the Plough (Ursa Major, the Great Bear) which is opposite the handle points away from the base towards the North Star, Polaris. This is not a particularly bright star, but it is in an uncluttered part of the sky so it's not hard to find. It is part of the unremarkable constellation Ursa Minor, and lies about three fifths of the way from the Plough to Cassiopeia (an easily-recognised crooked W). How high above the horizon it is depends on how far you are from the equator.

The Moon

Full moon and new moon each last for about three days. Every day the moon rises and sets about 30 to 70 minutes later than the day before.

The part of the moon which is lit up is always the portion nearest the sun, although due to an optical illusion it often looks like it points off to one side. How much of the moon is illuminated is in direct proportion to its 'distance' from the sun in the sky; at full moon, for instance, the sun is right over at the far side of the sky from the sun.

These two facts make it possible, in principle, to tell how far below the horizon the sun is just by looking at the moon. Every day the moon rises and sets about 30 to 70 minutes later than the day before.

Traditionally, the full moon makes people go a bit crazy (hence the word 'lunacy'). Scientists disagree on the veracity of this belief; some individual studies have found an effect, but wider overviews have dismissed their findings. Strangely, a recent study by BT found that people use the phone more in the days before a full moon.

The Planets
Mars and Venus are often among the brightest things in the sky; Mercury, Jupiter and Saturn can all be seen with the naked eye on a good night but are much less obvious.

Because Venus is closer to the sun than we are, it is never very far from the sun in the sky; so it is usually only visible in the morning and evening, and it never strays all that far from the horizon.

If you have good eyesight, it is possible to get the knack of telling the brighter planets from stars by the fact that they look slightly bigger, often with a sort of 'starburst' effect.

Meteors
Meteors are rocks heated to great temperatures as they enter our atmosphere. Every year at roughly the same times the Earth passes through rocky patches of our solar system which lead to meteor showers. The intensity of the showers, unfortunately, is almost totally unpredictable.

Comets
Comets are big balls of ice and rock in long orbits around the sun. Every now and then a really spectacular comet comes along, and is visible from Earth for days or weeks.

Satellites
These days many of the glowing dots we see in the sky are man-made; sometimes it's possible to see them drifting sedately along, betraying their artificial origin. The International Space Station is now the third brightest thing in the night sky, after the moon and Sirius.

A note on night vision: At low light levels, our vision depends almost entirely on our rod cells. This is why we have so little colour vision at night, and to see faint stars and the like we need to look off to one side a little - the tiny detailed spot at the centre of our visual field, the macula, is made up of cone cells, which provide tremendous acuity and distinguish colour but become useless at low light levels.