Drag the Moon around its orbit and see why it looks crescent, half or full — then switch to the side view to see why solar and lunar eclipses are rare.
The Moon's shape is not Earth's shadow. Half of the Moon is always lit by the Sun — what changes is the angle we see that lit half from. Earth's shadow only touches the Moon during a lunar eclipse.
Drag the Moon along its orbit
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Phase
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Moon age (days since new)
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Lit fraction
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Rise · highest · set (approx.)
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The tilt (really 5.1°) and distances are exaggerated for clarity. The verdict uses real figures.
Solar eclipse
Lunar eclipse
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Progress
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Score
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Streak
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Accuracy
Tonight's Moon
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Computed from the lunar phase algorithm in Meeus's Astronomical Algorithms, accurate to a few minutes. Times are in your device's time zone.
Upcoming eclipses 2027–2030
Source: NASA Goddard eclipse catalogue. Penumbral lunar eclipses are left out because they are barely visible.
The Moon does not shine on its own
Everything you see in the simulator follows from one fact: the Moon is a rock lit by the Sun. At any moment exactly half of it is in daylight and half in night, just like Earth. Sunlight arrives in parallel rays, so in the top view the lit half of the Moon always faces left, toward the Sun, no matter where the Moon is on its orbit.
What changes is our viewpoint. From Earth we look at that lit half from a different angle each night, so we see a different slice of it. That slice is the phase. The common idea that the dark part is Earth's shadow is wrong — Earth's shadow is a thin cone pointing away from the Sun, and the Moon only enters it during a lunar eclipse.
One lap through the eight phases
Drag the Moon counter-clockwise from the point nearest the Sun. At new Moon the night side faces us and nothing is visible. Over the next week a crescent grows on the right, reaching the half-lit first quarter when the Moon is 90° from the Sun. Past that the gibbous Moon swells until, opposite the Sun, we face the whole daylit half: full Moon. The second half of the lap runs in reverse — waning gibbous, last quarter lit on the left, waning crescent, and back to new.
The angle between Sun and Moon as seen from Earth is called elongation, and the simulator shows it under the big Moon. Elongation also tells you how much of the disc is lit: the fraction is (1 − cos θ) ÷ 2, which gives 0 at new, 50% at the quarters and 100% at full.
Why a crescent only shows in the evening (or at dawn)
A thin waxing crescent sits only 20–40° east of the Sun. By day it is lost in the glare; it becomes visible only when the Sun has set and the sky darkens, and because it follows the Sun across the sky it sets an hour or two later. That is why a new crescent is always a low, western, early-evening sight. The waning crescent is the mirror image: it leads the Sun, rising an hour or two before dawn in the east.
The rule generalises: the Moon rises roughly 50 minutes later each day. The simulator's rise, highest and set times use the simplest version of this — each 15° of elongation shifts the Moon one hour later than the Sun — so treat them as a guide, not a timetable; latitude and season move them by an hour or more.
From the southern hemisphere it looks upside down
Tick the southern-hemisphere box and the big Moon flips left to right. Nothing changes in space; the observer is standing the other way up on the globe, so the Moon's north is at the bottom of their view. In Buenos Aires, Santiago or Sydney the waxing crescent is lit on the left, and a textbook written for Europe or Korea shows the quarters the wrong way round. Near the equator the terminator runs almost horizontally and the crescent looks like a bowl.
29.5 days or 27.3 days? Two kinds of month
Measured against the stars, the Moon completes an orbit in 27.32 days, the sidereal month. But in that time Earth has moved about 27° along its own orbit, so the Sun's direction has shifted and the Moon needs roughly two more days to get back to the same phase. New Moon to new Moon takes 29.53 days on average, the synodic month, which is the one our calendars and the word “month” descend from.
The synodic month is only an average. Because the Moon's orbit is an ellipse and the Sun's pull varies, individual months range from about 29.3 to 29.8 days. “Tonight's Moon” on this page uses the full calculation rather than the average, which is why it can give new and full Moon dates to within minutes.
Solar eclipse: the Moon's shadow falls on Earth
At new Moon the Moon is between us and the Sun. If it lines up well enough, its shadow reaches Earth. The dark core of the shadow, the umbra, is tiny where it touches the ground — at most about 270 km across — so a total eclipse is seen only along a narrow track. Around it the lighter penumbra covers thousands of kilometres, where people see a partial eclipse with a bite taken out of the Sun.
By a remarkable coincidence the Sun is about 400 times wider than the Moon and about 400 times farther away, so the two discs look almost the same size. When the Moon is near the far point of its orbit it looks slightly smaller than the Sun, cannot cover it completely, and leaves a bright ring: an annular eclipse. The simulator's inset shows where the shadow lands on Earth as you move the date.
Lunar eclipse: the Moon enters Earth's shadow
At full Moon the geometry is reversed and Earth sits between Sun and Moon. Earth's umbra at the Moon's distance is about 2.7 times wider than the Moon, so the Moon can be swallowed whole for up to an hour and three quarters. Because the Moon itself is darkened, everyone on the night half of Earth sees the same eclipse at the same moment — no travel required, and it is perfectly safe to watch.
The Moon does not vanish but turns copper-red. Earth's atmosphere bends sunlight into the shadow and scatters away the blue, so what reaches the Moon is the light of every sunrise and sunset on Earth at once. A penumbral eclipse, when the Moon only crosses the outer shadow, is so subtle that most people never notice it.
Why there is no eclipse every month
If the Moon orbited in exactly the same plane as Earth's orbit, there would be a solar eclipse at every new Moon and a lunar eclipse at every full Moon. It does not: the Moon's orbit is tilted by 5.1°, which at the Moon's distance lifts it up to about 5 Earth radii above or below the shadow line. Most months the shadow simply misses.
The orbit crosses the ecliptic plane at two points, the nodes. An eclipse needs a new or full Moon while the Sun is lined up with the node line, which happens for about 34 days twice a year — the eclipse seasons. The side view shows this: as the date advances the orbit seems to tilt and flatten, and only when it is nearly edge-on to the Sun does the verdict turn green. Because the node line slowly drifts backwards, the seasons arrive about 19 days earlier each year, and the whole pattern repeats every 18 years and 11 days, the saros cycle known since Babylonian times.
Solar eclipse: new Moon + Sun within about 15° of a node (partial) or 10° (central).
Lunar eclipse: full Moon + Sun within about 11° of a node (umbral) or 17° (penumbral).
Each year has at least 2 solar eclipses and at most 7 eclipses in total.
No. The dark part is simply the Moon's own night side, the half facing away from the Sun. Earth's shadow is a narrow cone pointing away from the Sun, and the Moon only passes through it during a lunar eclipse, at most a few times a year. If the phases were caused by Earth's shadow, a crescent could never be seen near the Sun in the evening sky — yet that is exactly where it appears.
Why do we always see the same side of the Moon?
Because the Moon spins on its axis in exactly the time it takes to orbit Earth, about 27.3 days. This is called synchronous rotation, and it is the result of billions of years of tidal braking by Earth's gravity. The Moon does rotate — one lunar day lasts about a month — but it keeps the same face turned toward us. The simulator shows this: the little Moon on the orbit always keeps the same side toward Earth.
What are a supermoon and a blue moon?
A supermoon is a full Moon that happens when the Moon is near perigee, the closest point of its slightly oval orbit. It looks up to about 14% wider and 30% brighter than a full Moon at apogee, a difference that is real but hard to notice by eye. A blue moon is simply the second full Moon in one calendar month; it is not blue. Because the lunar cycle is 29.5 days, this happens about every two and a half years.
Why does a total solar eclipse never last more than about 7 minutes?
The Moon's umbra, the cone of full shadow, is at most about 270 km wide where it touches Earth, and it sweeps across the surface at well over 1,700 km/h because the Moon is moving along its orbit. Divide the width by the speed and you get a few minutes at best. The theoretical maximum is about 7 minutes 32 seconds; the eclipse of 2 August 2027 reaches 6 minutes 23 seconds, the longest of this century.
Why must you never look at a solar eclipse without a filter?
During a partial eclipse, any uncovered sliver of the Sun's surface is as bright as ever, and the retina has no pain receptors, so damage happens without warning. Sunglasses, smoked glass and camera film do not block enough infrared and ultraviolet. Use eclipse glasses that meet ISO 12312-2, a welder's glass of shade 14, or project the image through a pinhole. Only during the brief total phase, with the Sun fully covered, is it safe to look directly.
How accurate is “Tonight's Moon”?
The new and full Moon times come from the phase algorithm in Jean Meeus's Astronomical Algorithms, with its periodic and planetary corrections, which agrees with published almanacs to within a few minutes. The dates are shown in your device's time zone, so a full Moon near midnight may fall on a different calendar day than in a table printed for another country. The drawn shape uses the Moon's age, which is accurate to about a degree of phase angle.