Is the moon always visible to everyone on Earth

This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!

Why can I sometimes see the Moon in the daytime? – Sebastian, age 4, Brisbane.

Thank you, Sebastian, for your great question.

The Moon can be seen at different times during the day and night - but when and where you see it depends on the phase of the Moon.

If you watch the Moon over a month, you will notice it gradually changing shape, from a crescent to a full circle, then back to a crescent again. If you pay close attention, you will notice that its position in the sky also changes.

You’ve heard of sunrise and sunset, but did you know there’s also moonrise and moonset? Here are the moonrise and moonset times for Brisbane, where you live.

The time of moonrise and moonset and the shape of the Moon change throughout the month, depending on where the Moon is in space.

The Moon moves around the Earth every month. But while it’s doing that, it is also spinning like a toy top – we call that spinning bit “rotating on its axis”.

The time it takes the Moon to rotate on its axis is the same time it takes to complete one trip (or “orbit”) around the Earth. This is why we always see the same side of the Moon.

It also follows a similar path across the sky as the Sun. As it goes around the Earth, sunlight bounces off the Moon and toward us from different angles.

When the Moon is close to the Sun in the sky, the Sun is too bright for us to see it. As it moves away from the Sun in the sky each day, we see it emerge as a thin crescent. This called the New Moon.

If you look closely, you can see the faint glow of the “dark” part of the Moon. This is sunshine that is bouncing off of the Earth’s atmosphere back onto the Moon. Some Aboriginal groups see this as the spirit of the Moon-man.

Each day we see more of the Moon’s lit side as it gets bigger and bigger. When the moon rises at midday, the left half of it is lit up. We call this the First Quarter.

Each day, the Moon rises about 50 minutes later than it did the day before. When it’s a Full Moon, it rises at sunset.

When the Sun rises in the early morning, the Full Moon sets. This is the only phase when the Moon is in the sky all night.

The Moon starts to fade again. When it rises at midnight, only the right half of the Moon is lit up, which we call Last Quarter. It moves closer to the Sun each day, turning back to a crescent and fading away until it disappears. It stays “hidden” for three days before it emerges again as a New Moon.

Hawaiian people have a special name for every day of the lunar month. You can learn their song to help you remember!

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As a rule of thumb, a Full Moon can only be seen at night. So, why do our Moon Phase Charts show that some Full Moons occur in the daytime?

Moon phases are defined by the Moon's position, not its visibility.

©timeanddate.com

During the moment of the Full Moon, the Sun and the Moon are on opposite sides of the Earth, and the Moon's illuminated side faces the night side of Earth (see illustration). So, by definition, a Full Moon can usually only be seen during the night.

If you have ever spotted a Full Moon just after sunrise or just before sunset, it is very likely that you saw it at least one day before or after the actual Full Moon phase. Very rarely, if the conditions are just right, you may also be able to see an actual Full Moon very close to the horizon and opposite to the Sun, during sunrise or sunset. In general, however, the Moon is always below the horizon while the Sun is up on the date of the Full Moon.

So, why do our Moon Phase Charts show some Full Moons taking place in the middle of the day?

Moonrise and Moonset in my city

The Moon: Our natural satellite

Alignment Determines Moon Phase

The answer lies in the method astronomers use to determine the moment of each Moon phase. The phases of the Moon are not defined by the Moon's visibility from a particular location on Earth, but by the Moon's position on its orbit around Earth.

For example, the New Moon phase occurs at the exact moment when the Moon is between the Sun and the Earth, and the three bodies form a more or less straight line. Conversely, it is officially Full Moon when the Moon is aligned with the Sun and Earth while at the opposite end of its orbit, on the night side of Earth.

Astronomers call this alignment a syzygy of the Sun-Earth-Moon system.

Why lunar eclipses don't happen every Full Moon

A Full Moon in Lofoten, Norway. The Moon appears full for 1 or 2 nights around the Full Moon alignement.

©bigstockphoto.com/harvepino

Visibility Depends on Location

By this definition, the Full Moon alignment happens at one specific moment in time, irrespective of the time of day in any particular location. Since half of our planet always faces the Sun, a Full Moon will always occur during the day in some locations, while it is night in others. If your location happens to face the Sun at the moment of the alignment, our charts will show a Full Moon during the daytime—although you will not be able to see it at that precise moment.

What is the Moon Illusion?

Moon Looks Full for Days

While the Full Moon phase is technically a specific moment in time—the instant of the syzygy—the Moon looks fully illuminated for some days before and after the actual alignment, during the late stages of the Waxing Gibbous Moon and the beginning of the Waning Gibbous Moon.

Therefore, weather permitting, everyone on Earth will see a fully illuminated Moon in the nights surrounding the Full Moon alignment, even though only 98% or 99% of the visible half of the Moon may be illuminated.

Tip: Our Moon Phase charts show the actual illumination percentage for a moment of your choice and a realistic live animation of the current Moon phase.

How far is the Moon from Earth?