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Phases and Tides of the Moon Other than the Sun, the second-most obvious object in the sky is of course the Moon. The time between successive new Moons, called the synodic period, is about 29.5 days. Interestingly, the rotational period of the Moon relative to the Sun is exactly the same as the synodic period. Thus, from the Earth, we can only see one side - by definition the near side - of the Moon. This is called synchronous rotation. It is not a coincidence. The mutual gravity of the Earth and Moon locks Moon’s the revolution and rotational rates.

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Phases of the Moon

Since the Moon only shines by reflecting sunlight, the amount of its surface that is illuminated - its phase - varies at different times of the month. The cycle of phases starts from new moon to waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent, then to new moon again. The phases of the Moon. The moon rotates at a uniform rate, which is the same as its average orbital period around the Earth. But the moon’s orbit is not perfectly circular, so sometimes the moon is slightly ahead of its average position in its orbit. The moon’s orbit is also slightly inclined (tilted) to its rotational axis. Because of these facts, we can sometimes see a little bit more of one side of the Moon than the other. This phenomenon, called libration, lets us see just over 60% of the Moon’s surface, in total.

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The Tides

The tides are probably the most well-known phenomenon we experience on the Earth that is due to the influence of the Moon - werewolves notwithstanding! The Moon’s gravity exerts a stronger attractive force on the near side of the Earth, less at the center of the Earth, and even less on the far side. Relative to the center of the Earth, the effective forces pull water on the Earth both toward and away from the Moon. They lead to two bulges of water on the Earth’s surface. Therefore, we have two high tides per day. Why there are two tides per day. In addition to the Moon, the Sun also attracts water on Earth. When the Moon is new or full, the tidal effects from the Sun and Moon reinforce each other. The high tides at these times - called spring tides - will be higher than the high tides at other times. When the Moon is at first or last quarter, the tidal effects tend to cancel each other out, so the high tides are lowest then. These are called neap tides. Spring Tides (top) and Neap Tides (bottom). Tidal Evolution of the Earth and Moon Because the Earth is rotating, the tidal bulge does not point exactly toward the Moon. The Earth turns faster than the Moon orbits, so the Earth’s tidal bulge actually points slightly ahead of the Moon (by about 10°). The Moon’s gravity tugs back on the bulge, which very slowly causes the Earth’s rotation rate to slow down. Similarly, the gravity of the bulge tugs the Moon forward, which very slowly causes the Moon to speed up in its orbit. Due to this tidal interaction, the length of both the day and the month is slowly increasing, and the Moon is slowly receding from the Earth. Hundreds of millions of years from now, the Earth and Moon will both be “tide-locked” to each other: the Earth will rotate at exactly the same rate that the Moon orbits. The Earth will always keep the same face toward the Moon, just as the Moon, today, always keeps the same face toward the Earth. Tidal interactions slow the Earth’s rotation and lengthen the Moon’s orbital period. Just as the Moon raises tides on the Earth, the Earth’s gravity raises tides in the solid structure of the Moon. But because the Earth is much more massive than the Moon, the Earth tide-locked the Moon’s rotation billions of years ago. This is why the Moon always keeps the same face pointed toward the Earth. We encounter many other examples of such tidal locking in other pairs of objects in the Solar System.