Moon Phase Tracker.

Frequently asked questions.

How is the moon phase calculated?

We calculate the current lunar phase using the synodic period of 29.53058867 days — the average time from one new moon to the next. Starting from a known reference new moon (January 6, 2000 at 18:14 UTC, a well-established astronomical epoch), we compute how many complete synodic cycles have elapsed since then and find the fractional position within the current cycle. This gives us the moon's age in days (0 = new moon, ~14.77 = full moon, ~29.53 = next new moon), which is then converted to an illumination percentage and phase name. The calculation runs entirely in your browser and requires no internet connection or GPS.

What is the synodic month?

The synodic month (29.53 days) is the time it takes the moon to complete one full cycle of phases as seen from Earth — from new moon back to new moon. It is called 'synodic' because it is measured relative to the Sun (from the Greek 'synodos' meaning conjunction). This is different from the sidereal month (27.32 days), which is the time the moon takes to complete one full orbit of Earth measured relative to the fixed stars. The sidereal month is shorter because Earth itself moves along its orbit around the Sun during those 27 days — by the time the moon has completed one orbit, it needs to travel a little further to 'catch up' to the same position relative to the Sun, adding about 2.2 days to produce the longer synodic month.

Why does the moon look bigger near the horizon?

The moon illusion is one of the most studied perceptual phenomena in psychology, and despite centuries of study, the exact cause is still debated. What is certain is that it is entirely in your brain, not in the moon itself. When the moon is low on the horizon, surrounding objects — trees, buildings, hills — provide a size reference that makes the brain interpret the moon as large. High in the sky with no reference objects, the brain interprets it as smaller. You can confirm this is an illusion by holding a small object (like a pencil eraser) at arm's length and comparing its apparent size against the horizon moon and the overhead moon — they will be identical. The moon's actual angular diameter in the sky varies by about 14% across its orbit (larger near perigee, smaller near apogee) but this is independent of the illusion.

What is a supermoon?

The term supermoon (not an official astronomical term, but widely used) refers to a full moon that occurs when the moon is at or near perigee — its closest point to Earth in its elliptical orbit. At perigee, the moon is about 356,500 km away; at apogee (farthest point) it is about 406,700 km away — a difference of roughly 14%. A full moon at perigee appears about 14% wider in diameter and up to 30% brighter than a full moon at apogee (sometimes called a micromoon). Supermoons happen about 3–4 times per year because perigee and a full moon coincide every few cycles. The visual difference is real but subtle to the naked eye — most people cannot reliably tell a supermoon from an average full moon without a comparison.

Does the moon phase affect tides?

Yes, significantly. Tides are caused by the gravitational pull of the moon (and to a lesser extent the sun) on Earth's oceans. During full moon and new moon, the Sun, Earth, and Moon are roughly aligned — their gravitational forces act along the same line and combine, producing spring tides with unusually high high-tides and low low-tides. During the first and last quarter phases, the Sun and Moon are at 90° to each other relative to Earth — their tidal forces partially cancel, producing neap tides that are more moderate. The tidal range (difference between high and low tide) can be two or more times greater during spring tides than neap tides in some coastal locations. Coastal communities, sailors, fishermen, and surfers all plan around the tidal cycle using the lunar calendar.

The Eight Moon Phases — A Complete Guide

The moon does not produce its own light. What we see as the "moon" is sunlight reflecting off its surface, and because the moon orbits Earth over a cycle of 29.53 days (the synodic month), the angle between the moon, Earth, and Sun changes continuously. This changing angle determines how much of the sunlit half of the moon is visible to us — producing the eight distinct phases we recognise.

The eight phases in detail

🌑 New Moon — The moon is between Earth and the Sun, so the sunlit half faces away from us entirely. The moon is invisible from Earth (or barely visible as a dark disc). This is also the phase of solar eclipses when the alignment is precise enough. New moons are the darkest nights of the month, making them the best time for stargazing.

🌒 Waxing Crescent — A thin sliver of light appears on the right side (in the Northern Hemisphere) as the moon moves east of the Sun. The moon sets a few hours after the sun each evening, so it is visible low in the western sky after dusk.

🌓 First Quarter — Exactly half the visible face is lit, on the right side. The moon is 90° east of the Sun, so it rises around noon, is highest in the sky at sunset, and sets around midnight.

🌔 Waxing Gibbous — More than half the face is lit, but not yet full. The moon rises in the afternoon and is visible throughout much of the night, setting in the early morning hours.

🌕 Full Moon — The entire visible face is illuminated. The moon is directly opposite the Sun in the sky, so it rises at sunset and sets at sunrise. Full moons are the brightest nights of the month, which can wash out fainter stars and the Milky Way. Folklore has associated full moons with heightened emotion and unusual events for centuries — the word "lunatic" derives from the Latin luna (moon).

🌖 Waning Gibbous — The lit area begins to shrink from the right. The moon now rises after sunset, peaks in the sky after midnight, and sets in the morning. You can often see it in the western sky during the day.

🌗 Last Quarter — Half the face is lit again, now on the left side. The moon rises around midnight, is highest at dawn, and sets around noon — opposite to the First Quarter.

🌘 Waning Crescent — A thin sliver of light remains on the left side. The moon rises just before the sun and is visible low in the east in the predawn sky. This phase closes the cycle, leading back to the next New Moon.

How the moon affects tides

The moon's gravity pulls on Earth's oceans, creating a bulge of water on the side closest to the moon and (due to inertia) a corresponding bulge on the far side. As Earth rotates under these bulges, most coastal locations experience two high tides and two low tides every 24 hours and 50 minutes (a lunar day, slightly longer than a solar day because the moon moves in its orbit while Earth rotates).

During Full Moon and New Moon, the Sun, Earth, and Moon are aligned. The Sun's tidal force adds to the Moon's, producing larger spring tides — higher high tides and lower low tides than average. During First and Last Quarter, the Sun and Moon are at 90° and their tidal forces partially cancel, producing smaller neap tides. This cycle is why coastal tides vary so dramatically over a two-week period, and why fishermen, surfers, and harbour pilots pay close attention to the lunar calendar.

Supermoons and micromoons

The moon orbits Earth in a slightly elliptical path, not a perfect circle. Its closest point is called perigee and its farthest point is apogee. When a Full Moon occurs near perigee, it is called a supermoon — it can appear up to 14% larger in diameter and about 30% brighter than a Full Moon at apogee. When a Full Moon occurs near apogee, it is sometimes called a micromoon and appears smaller and dimmer than average. The difference is subtle to the naked eye, but it is real and measurable.

Best times to stargaze by moon phase

The moon is the biggest source of light pollution that observers have no control over. A bright Full Moon can raise the background sky brightness enough to wash out galaxies, nebulae, and faint stars entirely. The best nights for deep-sky observing — the Milky Way, star clusters, and galaxies — are the days around New Moon when the sky is truly dark all night. The nights around First and Last Quarter offer a useful compromise: the moon sets (or rises) late enough to give several hours of dark sky. Use this tracker to plan your next observing session around the lunar calendar.