Some nights when we look up at the moon, it is full and bright; sometimes it is just a sliver of silvery light. These changes in appearance are the phases of the moon. As the moon orbits Earth, it cycles through eight distinct phases. The four primary phases occur about a week apart.

Here are the dates of the moon's phases for 2018, according to NASA. Times and dates are in Eastern U.S. time.

1st Qtr Full Moon Last Qtr New Moon
    Jan 1 21:24 Jan 8 17:25 Jan 16 21:17
Jan 24 17:20 Jan 31 08:27 Feb 7 10:54 Feb 15 16:05
Feb 23 03:09 Mar 1 19:51 Mar 9 06:20 Mar 17 09:12
Mar 24 11:35 Mar 31 08:37 Apr 8 03:18 Apr 15 21:57
Apr 22 13:49 Apr 29 20:58 May 7 22:09 May 15 07:48
May 21 23:49 May 29 10:20 Jun 6 14:32 Jun 13 15:43
Jun 20 06:51 Jun 28 00:53 Jul 6 03:51 Jul 12 22:48
Jul 19 15:52 Jul 27 16:21 Aug 4 14:18 Aug 11 05:58
Aug 18 03:49 Aug 26 07:56 Sep 2 22:37 Sep 9 14:01
Sep 16 19:15 Sep 24 22:53 Oct 2 05:45 Oct 8 23:47
Oct 16 14:02 Oct 24 12:45 Oct 31 11:40 Nov 7 11:02
Nov 15 09:54 Nov 23 00:39 Nov 29 19:19 Dec 7 02:20
Dec 15 06:20 Dec 22 12:49 Dec 29 04:34    

The moon, like Earth, is a sphere, and it is always half-illuminated by the sun. However, as the moon travels around Earth, we see more or less of the illuminated half. The moon's phases describe how much of the moon's disk is illuminated from our perspective.

New moon: The moon is between Earth and the sun, and the side of the moon facing toward us receives no direct sunlight; it is lit only by dim sunlight reflected from Earth.

Waxing crescent: As the moon moves around Earth, the side we can see gradually becomes more illuminated by direct sunlight.

First quarter: The moon is 90 degrees away from the sun in the sky and is half-illuminated from our point of view. We call it "first quarter" because the moon has traveled about a quarter of the way around Earth since the new moon.

See the moon phases, and the difference between a waxing and waning crescent or gibbous moon, in this Space.com infographic about the lunar cycle each month. <a href="http://www.space.com/62-earths-moon-phases-monthly-lunar-cycles-infographic.html">See the full infographic</a>.
See the moon phases, and the difference between a waxing and waning crescent or gibbous moon, in this Space.com infographic about the lunar cycle each month. See the full infographic.
Credit: Karl Tate, SPACE.com

Waxing gibbous: The area of illumination continues to increase. More than half of the moon's face appears to be getting sunlight.

Full moon: The moon is 180 degrees away from the sun and is as close as it can be to being fully illuminated by the sun from our perspective. The sun, Earth and the moon are aligned, but because the moon’s orbit is not exactly in the same plane as Earth’s orbit around the sun, they rarely form a perfect line. When they do, we have a lunar eclipse as Earth's shadow crosses the moon's face.

Waning gibbous: More than half of the moon's face appears to be getting sunlight, but the amount is decreasing.

Last quarter: The moon has moved another quarter of the way around Earth, to the third quarter position. The sun's light is now shining on the other half of the visible face of the moon.

Waning crescent: Less than half of the moon's face appears to be getting sunlight, and the amount is decreasing.

Finally, the moon is back to its new moon starting position. Now, the moon is between Earth and the sun. Usually the moon passes above or below the sun from our vantage point, but occasionally it passes right in front of the sun, and we get a solar eclipse. [Infographic: How Moon Phases Work]