Moon Phase and Libration, 2024 South Up

Click on the image to download a high-resolution version with feature labels and additional graphics. Hover over the image to reveal the animation frame number, which can be used to locate and download the corresponding frame from any of the animations on this page, including unlabeled high-resolution Moon images.

The animation archived on this page shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year 2024, at hourly intervals. Until the end of 2024, the initial Dial-A-Moon image will be the frame from this animation for the current hour.

More in this series:
Moon Phase and Libration Gallery

The Moon's Orbit

From this birdseye view, it's somewhat easier to see that the phases of the Moon are an effect of the changing angles of the Sun, Moon and Earth. The Moon is full when its orbit places it in the middle of the night side of the Earth. First and Third Quarter Moon occur when the Moon is along the day-night line on the Earth. The Sun's direction is indicated by the yellow arrow.

The view here is perpendicular to the plane of the Earth's orbit around the Sun, called the ecliptic. The teal-colored ring is the plane of the Moon's orbit around the Earth, which is tilted about five degrees to the ecliptic. The thickness of the ring shows the range of the Moon's distance, and the darker half is the part above (north of) the ecliptic. The two points where the orbit crosses the ecliptic are the ascending and descending nodes. Also labeled are the perigee and apogee, the points along the orbit that are nearest to and farthest from Earth.

The First Point of Aries is at the 3 o'clock position in the image. The Sun is in this direction at the March equinox. You can check this by freezing the animation at around the 1:03 mark, or by freezing the full animation with the time stamp near March 20. This direction serves as the zero point for both ecliptic longitude and right ascension.

The south pole of the Earth is tilted 23.5 degrees toward the 12 o'clock position at the top of the image. The tilt of the Earth is important for understanding why the north pole of the Moon seems to swing back and forth. In the full animation, watch both the orbit and the "gyroscope" Moon in the lower left. The widest swings happen when the Moon is at the 3 o'clock and 9 o'clock positions. When the Moon is at the 3 o'clock position, the ground we're standing on is tilted to the left when we look at the Moon. At the 9 o'clock position, it's tilted to the right. The tilt itself doesn't change. We're just turned around, looking in the opposite direction.

The subsolar and sub-Earth points are the locations on the Moon's surface where the Sun or the Earth are directly overhead, at the zenith. A line pointing straight up at one of these points will be pointing toward the Sun or the Earth. The sub-Earth point is also the apparent center of the Moon's disk as observed from the Earth.

In the animation, the blue dot is the sub-Earth point, and the yellow cone is the subsolar point. The lunar latitude and longitude of the sub-Earth point is a measure of the Moon's libration. For example, when the blue dot moves to the left of the meridian (the line at 0 degrees longitude), an extra bit of the Moon's eastern limb is rotating into view, and when it moves above the equator, a bit of the far side beyond the south pole becomes visible.

At any given time, half of the Moon is in sunlight, and the subsolar point is in the center of the lit half. Full Moon occurs when the subsolar point is near the center of the Moon's disk. When the subsolar point is somewhere on the far side of the Moon, observers on Earth see a crescent phase.

The Moon's orbit around the Earth isn't a perfect circle. The orbit is slightly elliptical, and because of that, the Moon's distance from the Earth varies between 28 and 32 Earth diameters, or about 356,400 and 406,700 kilometers. In each orbit, the smallest distance is called perigee, from Greek words meaning "near earth," while the greatest distance is called apogee. The Moon looks largest at perigee because that's when it's closest to us.

The animation follows the imaginary line connecting the Earth and the Moon as it sweeps around the Moon's orbit. From this vantage point, it's easy to see the variation in the Moon's distance. Both the distance and the sizes of the Earth and Moon are to scale in this view. In the HD-resolution frames, the Earth is 50 pixels wide, the Moon is 14 pixels wide, and the distance between them is about 1500 pixels, on average.

Note too that the Earth appears to go through phases just like the Moon does. For someone standing on the surface of the Moon, the Sun and the stars rise and set, but the Earth doesn't move very much in the sky. It goes through a monthly sequence of phases as the Sun angle changes. The phases are the opposite of the Moon's. During New Moon here, the Earth is full as viewed from the Moon.

The Named Phases