Moon phases and light at night

The sun always lights exactly one half of the moon, just as it always lights one half of the Earth – where it is day. The moon orbits the Earth, and as it does so we see from Earth a changing share of its lit half. When the moon is between the sun and Earth, it shows us its dark side: new moon. When it is on the side opposite the sun, we see the whole lit half: full moon.

A complete cycle from new moon to new moon takes about 29.5 days. In between come the first quarter, about a week after new moon, and the last quarter, about a week after full moon. Because the moon moves a little further along its orbit each day, the Earth has to turn a little longer each day until it rises at the same point again – on average about 50 minutes.

This gives a simple timetable: the full moon rises around sunset and sets around sunrise. The first-quarter moon is already in the sky in the afternoon and evening and sets around midnight. The last-quarter moon rises around midnight and can still be seen in the morning. The thin crescent shortly before or after new moon is visible only briefly at twilight near the sun.

Our eyes have two kinds of light-sensing cells. Cones see colour and sharp detail but need a lot of light. Rods are very light-sensitive but do not see colour and see less sharply. In moonlight it is mainly the rods that work. That is why a moonlit landscape looks silvery grey, colours disappear and fine edges are hard to make out.

The eyes take about half an hour to adapt largely to darkness. A single glance into bright light or at a bright screen undoes much of this adaptation. Dimmed red light disturbs it less, because rods respond only weakly to red light. That is why many headlamps have a red-light mode.

One trick helps you make out faint things: look slightly to one side of the object. In the centre of the field of view, where vision is sharpest, there are almost only cones. The sensitive rods lie further out. So if you look just beside a target, you often see it better in the dark.

For night walks, early starts or setting up camp in the dark, the days around full moon are the brightest. Check the moon phase and moonrise beforehand in a calendar, weather app or astronomical service. What matters is not only the phase but also whether the moon is above the horizon at all at the time you need it.

Clouds can ruin the best plan. Forest, narrow valleys and slopes facing away from the moon also lie in moon shadow and can be very dark, even at full moon. Snow, on the other hand, reflects a lot of light and makes moonlit nights especially bright.

At mid-latitudes the full moon stands high in the sky in winter and low in summer – exactly the opposite of the sun, because it is opposite the sun. Long winter nights with a full moon are therefore often surprisingly bright, summer nights with a full moon less so.

Moonlight is deceptive. Paths and large shapes are easy to see, but steps, roots, holes, wet stones and above all drop-offs disappear in the grey light. Shadows can fake deep holes and hide real drops. So always carry a lamp and spare batteries, even if you mainly want to walk by moonlight.

Plan night activities with a buffer: clouds can come in, the moon can disappear behind a ridge, and the way can take longer than expected. Tell someone where you are going and when you plan to be back. In an emergency, at night too: the local emergency number (112 in the EU).

For orientation the moon is only a rough aid. Like the sun it rises roughly in the east and sets in the west, but its rising and setting points shift a lot. For a reliable direction you need a map and compass.

A common misconception is that the moon's phases are caused by the Earth's shadow. That is not true – the phases result only from our viewing angle on the lit half of the moon. The Earth's shadow falls on the moon only during a lunar eclipse, and that can only happen at full moon.

In the northern hemisphere the waxing moon is lit on the right, in the southern hemisphere on the left. The phases themselves, however, happen at the same time everywhere on Earth. And the same alignment of sun and moon that produces full and new moon also causes particularly strong tides, the spring tides – more in the tides entry.