Understand how the Sun, Earth, and the Moon line up during eclipses, why eclipses are rare, and how the Moon raises the oceans.
When shadows line up
Two kinds of eclipses
An eclipse is when one celestial body blocks another's light or falls into a shadow.
Solar eclipse: the Moon moves exactly between the Sun and Earth and covers the Sun. For a short while it gets dark in the daytime. This happens only at new moon.
Lunar eclipse: Earth moves between the Sun and the Moon, and Earth's shadow falls on the Moon. The full Moon often turns reddish — it is lit by all of Earth's sunsets and sunrises at once. This happens only at full moon.
So why don't eclipses happen every month? Because the Moon's orbit is slightly tilted. Usually the Moon passes a little above or a little below the Sun in the sky, and the shadows miss. A “direct hit” happens only a few times a year.
An amazing coincidence: the Sun is about 400 times larger than the Moon, but also about 400 times farther away. That's why in our sky they look almost the same size — and the Moon can exactly cover the Sun's disk.
Lesson notes
Two kinds of eclipses
An eclipse is when one celestial body blocks another's light or falls into a shadow.
Solar eclipse: the Moon moves exactly between the Sun and Earth and covers the Sun. For a short while it gets dark in the daytime. This happens only at new moon.
Lunar eclipse: Earth moves between the Sun and the Moon, and Earth's shadow falls on the Moon. The full Moon often turns reddish — it is lit by all of Earth's sunsets and sunrises at once. This happens only at full moon.
So why don't eclipses happen every month? Because the Moon's orbit is slightly tilted. Usually the Moon passes a little above or a little below the Sun in the sky, and the shadows miss. A “direct hit” happens only a few times a year.
An amazing coincidence: the Sun is about 400 times larger than the Moon, but also about 400 times farther away. That's why in our sky they look almost the same size — and the Moon can exactly cover the Sun's disk.
Twice a day, the ocean level at the shore rises and falls — these are high and low tides. The main culprit is the Moon (and partly the Sun).
The Moon pulls on the water on the side of Earth nearest to it more strongly than on the planet itself, and the water there swells into a bulge. A second bulge forms on the opposite side of Earth — there the water seems to “lag behind.” The result is two high tides on opposite sides of the planet.
Earth rotates beneath these bulges, so in a day the shore passes through both, and high tides come about twice a day.
The strongest tides come at new moon and full moon, when the Sun and the Moon pull along the same line and combine their efforts.