National Astronomical Observatory of Japan

Year’s Smallest Full Moon (July, 2018)

A diagram showing the geocentric distance of full moons in 2017 and 2018
Medium resolution (2000 x 1265) High resolution (5500 x 3480)

Look up at the Moon, and See the Smallest Full Moon of 2018!

The full moon of July 28 is the smallest of all the full moons (Note 1) in 2018.

The Moon moves around the Earth. Since the Moon's orbit is not a circle but an ellipse, the distance between the Earth and the Moon is not constant. Also, the orbit of the Moon changes slightly as it is affected by the gravity of the Sun and the Earth, so that the distance when the Moon is at its closest position to the Earth (perigee), and the distance when the Moon is at its farthest position from the Earth (apogee) are different each time as shown in the figure above. The geocentric distance (the distance from the center of the Earth to the center of the Moon) during the full moon varies from approximately 356,000 kilometers to 406,000 kilometers. The apparent size of the Moon (apparent diameter (Note 2)) is large when the Earth and Moon are close; and small when they are far from each other. The apparent diameter of the largest full moon is 14 percent larger than that of the smallest full moon. Also the largest full moon appears 30 percent brighter than the smallest full moon.

The full moon of January 2 was the largest in 2018. The Moon passed through the perigee at 6:49 a.m. After that, it became full phase at 11:24 a.m. At the time of the full moon, the geocentric distance was about 357,000 kilometers, and the Moon’s apparent diameter was 33 arcminutes 30 arcseconds.

The full moon of July 28 is the smallest and furthest in 2018. The Moon passes through the apogee at 2:44 p.m. on July 27 and will be at full phase at 5:20 a.m. on July 28. The geocentric distance at the time of the full moon is about 406,000 kilometers, and the apparent diameter is about 29 minutes 25 arcseconds.

A diagram comparing the largest and smallest full moons of 2018
Medium resolution (2000 x 1265) High resolution (5500 x 3480)

You cannot line up the real moons in the night sky to compare the sizes. But if you could line up the largest full moon and the smallest full moon in 2018 as in the figure above, you could easily see the size difference.

This year, a total lunar eclipse occurs on the same day that the smallest full moon occurs.

(Note 1) “Full moon” in astronomy refers to the Moon at the moment when it is in the opposite direction from the Sun, as seen from the center of the Earth. Back

(Note 2) The apparent diameters shown on this page are calculated based on the geocentric distance. Back

Reference: Ephemeris Computation Office

With the “Sky Viewer” you can easily explore the appearance of a typical urban night sky (planets and constellations are visible). The Celestial Phenomena section of the glossary explains the planetary phenomena terms: greatest elongation, opposition, conjunction, stationary, etc.