Moon Tonight

How to use it

Tonight's Moon. When the page opens, the big Moon shows how the Moon looks tonight at 8 pm in your time zone. Next to it you will see:

• the phase name, such as Waxing crescent or Full moon
• how much of the Moon's face is lit, as a percentage
• the Moon's age in days, which is the time since the last new Moon
• whether it is waxing (getting bigger each night) or waning (getting smaller)
• the dates and times of the next new Moon, first quarter, full Moon and last quarter, in your own time zone

Scrub through 30 days. Drag the slider to move forward one day at a time and watch the light grow and shrink. Tap Play to watch it as a movie. If your device is set to reduce motion, there is no Play button, and the slider still works.

Pick a date. Use the date box, or tap any day in the calendar. Every day has its own little Moon, and a dot marks the days of the four main phases. Use the arrows to change month. The tool covers the years 1900 to 2099, so you can try your birthday.

Change your view. The Moon starts as it looks from the southern hemisphere, where Australia is. Switch on See it from the northern hemisphere to see it the way children in countries such as England or Canada see it.

The Moon is shown as it is at 8 pm. You can only see it at that time if it is above the horizon. A waxing Moon is up in the evening, but a waning Moon rises later in the night, so look for it in the morning before sunrise.

Why it works

The Moon makes no light of its own. The Sun always lights up half of it, the way a lamp lights up half of a ball. As the Moon travels around Earth, we see more of the lit half, or less of it. These changing shapes are the Moon's phases. Our explainer Why does the Moon change shape? tells the whole story.

The Moon takes about 27.3 days to go once around Earth, but the cycle of phases takes longer: about 29.5 days. Earth is moving around the Sun at the same time, so the Moon needs a couple of extra days to catch up. The average time from one new Moon to the next is called the synodic month. It is 29.530588853 days, which is about 29 days, 12 hours and 44 minutes.

Waxing means growing, and waning means shrinking. A crescent is less than half lit, and a gibbous Moon is more than half lit. The first quarter and last quarter Moons are called quarters because the Moon is a quarter and three quarters of the way round its orbit. They look like half circles.

Everyone on Earth sees the same phase at the same time, but the Southern Hemisphere sees the Moon the other way up. From Australia, a waxing Moon is lit on the left and a waning Moon is lit on the right. In the north it is the other way round. That is why the tool draws the dark patches on the Moon upside down for the southern view.

This tool works the Moon out for itself, with no internet needed. It uses the average length of the month and then adds corrections from a method by the astronomer Jean Meeus, from his book Astronomical Algorithms. The corrections are needed because the Moon's orbit is not a perfect circle, so it speeds up and slows down. The times of new, quarter and full Moons match published tables to within a few minutes. To find how much of the Moon is lit, the tool works out the angle between the Moon and the Sun in the sky.

The dark part of the Moon is not Earth's shadow. It is the Moon's own night side. Earth's shadow falls on the Moon only during a lunar eclipse.

For parents and teachers

The Moon Tonight tool is written for Years 3 to 8. Moon phases are usually taught in the upper primary and lower secondary years, so ask your child's teacher what the class is learning. Younger children enjoy the slider and the calendar. Older students can work with the age in days and the percentage lit.

Ideas to try:

• Start a Moon diary. Draw the Moon each evening for a month, then check your drawings against the calendar.
• Make a model. In a dark room, use a lamp as the Sun and a ball on a stick as the Moon. Hold the ball at arm's length and slowly turn on the spot, to see all of the phases appear.
• Ask Will the Moon be full on your birthday? Pick the date and see.
• Use the slider to find how many days it takes to go from one full Moon to the next.

The picture shows the phase, but not the tilt. In the real sky a crescent leans over by an amount that depends on the time of night and where you are, so the Moon outside may not match the drawing exactly. The size of the Moon in the sky also changes a little during the month, because its orbit is not a perfect circle. The tool does not show that.

Dates and times are for your device's time zone. Dates in the future are predictions that come from the same calculations, and they stay very accurate.

The tool works with touch, mouse and keyboard, and every day in the calendar has a label that tells a screen reader the date, the phase and how much is lit. The only thing the tool remembers on this device is which hemisphere you picked. It is not sent anywhere.