The Moon

The moon makes no light of its own. We see whatever part of its sunlit half happens to face us. As the moon orbits Earth, that visible slice changes. The cycle from new moon back to new moon takes about 29.5 days. Starting at new moon, the lit portion grows, which is called waxing. It moves through crescent, first quarter, and gibbous to full. At full moon the moon sits opposite the sun in our sky and rises around sunset. Then the lit portion shrinks, which is called waning, back through gibbous, last quarter, and crescent to new.

Here is one more detail worth knowing. The moon rotates exactly once per orbit, so it keeps the same face toward Earth all the time. Nobody on Earth saw the far side until a spacecraft flew around it in 1959. The whole cycle is completely predictable, which makes it a good example of cyclic change. Calendars have been built on it for thousands of years.

Slide through one 29.5-day lunar cycle. The lab names the phase, estimates how much of the visible face is lit, and tells you whether the moon is waxing or waning. Note which two phases make eclipses possible.

Eclipses happen when the sun, Earth, and moon line up exactly. In a lunar eclipse, the full moon passes through Earth's shadow. Everyone on the night side of the planet can see it at once, and the moon often glows coppery red as sunlight bends through Earth's atmosphere and lands on it. A solar eclipse works the other way around. The new moon's small shadow sweeps across Earth and briefly turns day to night along a narrow path only a few hundred kilometers wide. That is why a total solar eclipse is so rare from any one town, while a lunar eclipse is visible to half the world.

So why not an eclipse every month, at every new and full moon? Because the moon's orbit is tilted about 5 degrees to Earth's orbit around the sun. Most months the moon passes a little above or below the exact line, and the shadows miss. Only when a new or full moon happens near the two points where the orbits cross do the three bodies line up closely enough for an eclipse. That tilt is the whole reason eclipses are events rather than routine.

The moon's gravity also shapes the oceans. It pulls Earth's water into two bulges, one on the side facing the moon and one on the far side. As the planet rotates through those bulges, most coasts pass through two high tides and two low tides a day. The sun adds its own smaller pull. When the sun, Earth, and moon line up at new and full moon, the two pulls combine and you get the extra-large spring tides. When the sun pulls at a right angle near the quarter moons, the two pulls partly cancel and you get the weaker neap tides. Phases, eclipses, and tides all come from the same orbiting geometry.

The moon shines by reflected sunlight. As it orbits Earth every 29.5 days, we see changing slices of its lit half. Those are the phases, waxing from new to full and waning back, with the same face toward us the whole time. Exact alignments make eclipses, lunar at full moon and solar at new moon. The 5-degree tilt of the moon's orbit is what keeps them rare. The moon's gravity drags two tidal bulges around Earth, which gives most coasts two tides a day. Those tides are largest at new and full moon, when the sun pulls in line with the moon. Phases, eclipses, and tides are the clearest cyclic changes in the sky, and they can be predicted centuries ahead. The gravity that runs them is the subject of the next chapter.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.