Cometary Science Archive

The Earth's Moon

To introduce concepts of astronomy, it is often best to start with actual observing projects to engage the student/pupil in visualizing the solar system that the earth is a part of. The most obvious starting place for this is to look at what causes our days and nights (the earth rotating on its axis once every 24 hours from west toward the east, so that the sun appears to rise in the east and set in the west), and therefore look at both the sun and the moon.

The moon's apparent motion on the sky. The moon also rises in the east and sets in the west (as do the planets, comets, and the stars), but the moon varies quite a bit in rising and setting times (with respect to the sun) because the moon is also orbiting the earth (making a complete circuit around the earth in just under a month). One can see the moon's movement across the sky in only minutes when it is near the horizon (rising or setting), and noticeably over the course of an hour when it is higher in the sky -- as the earth is turning on its axis.

It takes the moon about 29.5 days to go from new moon to new moon -- the time that the moon is closest to the sun in the sky. When the moon is closest to the sun, it is invisible from the ground -- lost in the sun's glare, with our looking at the dark side of the moon (the sunlit part facing toward the sun and away from us. As the moon moves eastward around 13 to 15 degrees per day in the sky in its orbit about the earth (there are 360 degrees around the whole sky for it to complete its orbit in 27.3 days), it moves with respect to the background stars noticeably each day, causing it to rise nearly an hour later each day/night when compared to the previous day/night. Note that the moon is bright enough to be visible in broad daylight through most of its orbit around the earth.

Lunar phases. The motion of the moon around the earth over a month permits us to view the moon from different angles with respect to the sun. As it emerges from the sun's glare in the evening sky after new moon, it appears for a few days as a crescent moon. After about a week, it reaches "first quarter" phase, where it appears half lit (the other half that is lit is facing out into space, away from the earth).

After yet another week, the moon comes to "opposition", where it is opposite the sun in the sky, so that we see it fully lit ("full moon"); the moon is up all night long around full moon. After that, as it moves into the morning sky, less and less of the moon appears lit as each day/night passes, so that about a week after full moon, it reaches half-lit phase again (called "last quarter" moon). And a week after that, it has made full-circle to reach new moon again, after passing through several days of appearing as a crescent moon in the morning sky.

Good diagrams that show lunar phases with respect to the moon's position with respect to the earth and the sun in space can be viewed by clicking on the links here and here. A list of dates of lunar phases for the years 2001-2025 is given at this NASA website.

Eclipses. Also, the moon's orbit is inclined with respect to the earth's orbit about the sun, so that only occasionally (a couple of times a year) is the moon exactly lined up in space with the sun and the earth; when this happens, an eclipse occurs. A solar eclipse happens at new moon, when the moon crosses in front of the sun (with the moon appearing black because of the relative brightness of the sun). A lunar eclipse happens at full moon, when the earth passes between the moon and the sun (and so the earth's shadow is cast onto the moon's surface, and we see the moon getting darker and then brighter again over a couple of hours).

A list of lunar eclipses observable for the years 2008-2015 is given at this NASA website.

The moon close-up. The moon appears uneven in shading and texture to the naked eye. With binoculars, mountains and craters become visible; the moon has no atmosphere, so no clouds to hide its surface. The larger the optical instrument (telescopes), the more detail on the lunar surface that can be seen. The first spacecraft visited the moon in the 1960s, and since then we have mapped the entire surface with high-resolution photography (and brought back lunar samples via the several Apollo manned landings on the moon).

A good website for close-up lunar maps and images can be seen by clicking here. A nice gallery of lunar images can be found by clicking here. Another good gallery of images of the moon can be found here.

Detailed technical information on the moon, including data concerning its size, mass, etc., are given at this NASA website.

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