The Universe and the Solar System
Senior 5 · Geography
The Universe and the Solar System
Lesson 3 of 3
25 min
Look at the Moon on a few evenings in a row and its shape changes: a thin curve low in the west after sunset, a half-disc a week later, a full disc rising at sunset after two weeks. Calendars have been built on it for thousands of years, and it raises the ocean tides that ships entering East African ports such as Mombasa and Dar es Salaam must plan around.
Definition
Moon: the Earth's only natural satellite. It is about 3,475 km across (just over a quarter of the Earth's diameter), orbits at a mean distance of about 384,400 km, has no atmosphere and no liquid water, and its gravity is about one-sixth of the Earth's (NASA). Its surface has dark, smooth plains of old lava (maria), bright highlands and countless craters.
The Moon turns once on its axis in exactly the time it takes to orbit the Earth (27.3 days), so it always keeps the same face towards us. At any moment the half facing the Sun is lit. As the Moon goes round the Earth we see different amounts of that lit half: these are its phases.
From new moon to full moon the lit part grows (waxing); from full to new it shrinks (waning). Seen from Rwanda, so close to the equator, a crescent moon often appears to lie on its back like a boat, because the Sun is then almost straight below it.
Definition
Sidereal month: the time for the Moon to complete one orbit measured against the stars, 27.3 days. Synodic month (lunar month): the time from one new moon to the next, 29.5 days. Example: the Islamic calendar uses the synodic month, which is why Ramadan begins about 11 days earlier each year.
Worked example 1: why the lunar month is longer than one orbit
Problem. Show that a sidereal month of 27.32 days gives a synodic month of about 29.53 days.
Solution. While the Moon orbits, the Earth moves along its own orbit, so the Moon must travel a little further to line up with the Sun again. In one day the Moon turns through and the Earth–Sun line through . The Moon gains on the Sun line by the difference, so a full lap of 360° takes days where
A solar eclipse happens when the Moon passes directly between the Sun and the Earth, so it can occur only at new moon. Only a narrow strip of the Earth lies in the Moon's dark shadow (the umbra) and sees a total eclipse; a much wider area in the partial shadow (the penumbra) sees a partial one. When the Moon is near its farthest from the Earth it looks too small to cover the Sun, and a ring of sunlight remains: an annular eclipse. A lunar eclipse happens when the Earth passes between the Sun and the Moon, so it can occur only at full moon; the whole night side of the Earth can see it, and the Moon often turns a dull red, lit by sunlight bent through the Earth's atmosphere.
Eclipses do not happen every month because the Moon's orbit is tilted about 5° to the Earth's. Usually the new or full moon passes just above or below the Sun–Earth line.
The Moon's gravity pulls the ocean on the side nearest it into a bulge; a second bulge forms on the far side, where the Moon's pull is weakest and the Earth is, in effect, pulled away from the water. As the Earth turns beneath the two bulges, most coasts have two high tides and two low tides in about 24 hours 50 minutes (the extra 50 minutes because the Moon has moved on in its orbit). The Sun raises smaller tides too. At new and full moon, when Sun, Moon and Earth are in line, their pulls add to give spring tides, with the highest high tides and lowest low tides. At first and last quarter they pull at right angles, giving neap tides with the smallest range.
Definition
Asteroid: a small rocky or metallic body orbiting the Sun, most of them in the asteroid belt between Mars and Jupiter. Example: Ceres, about 940 km across, the largest. Comet: a body of ice and dust, a few kilometres across, that grows a glowing head (coma) and tails when its orbit brings it near the Sun. Example: Halley's Comet, which returns about every 76 years, last in 1986 and next in 2061.
Meteoroid: a small piece of rock or metal in space. Meteor: the streak of light ("shooting star") when a meteoroid burns up in the atmosphere. Meteorite: a piece that survives the fall and reaches the ground. Example: the Mbale meteorite, which fell in eastern Uganda on 14 August 1992.
Most meteoroids are grains of dust that burn up 80 to 120 km above the ground. Large impacts are rare but important: an asteroid about 10 km across struck what is now Mexico about 66 million years ago and is linked to the extinction of the dinosaurs. The largest meteorite ever found, the Hoba meteorite in Namibia, weighs about 60 tonnes and still lies where it fell.
Warning
Common mistakes
Tip
Memory hook. Spring tides have nothing to do with the season: think of the water "springing up" when the Sun and Moon pull together at new and full moon.
CrashCourse: "Moon Phases: Crash Course Astronomy #4". Watch how the angle between the Sun, the Earth and the Moon sets the phase, then check each phase against Figure 4.
Practice
Summary
Further reading
Sources and credits
Written by Ganzaa. Moon data: NASA Moon Fact Sheet and NASA Science. Figures 4 to 6 were drawn by Ganzaa.
Photo: "Comet Hale-Bopp 1995O1", E. Kolmhofer and H. Raab, Johannes Kepler Observatory, Linz, Wikimedia Commons, CC BY-SA 3.0, unaltered, caption by Ganzaa. This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
Video: "Moon Phases: Crash Course Astronomy #4", CrashCourse, embedded from YouTube.
