Earth, Moon, and Sun

In survival games, night means monsters, so you'd better know when the Sun goes down. In real life, the day-night cycle, the seasons, the Moon's phases, eclipses, and tides all come from how three objects move: Earth, the Moon, and the Sun.

Earth moves in two ways at once:

• Rotation: Earth spins on its axis, an imaginary line through the North and South Poles. One spin takes about 24 hours and gives us day and night. At the equator, you're spinning at about 1,600 km/h (1,000 mph). You just don't feel it.
• Revolution: Earth travels around the Sun in a path called an orbit. One trip takes about 365¼ days, which is one year. That extra quarter day is why we add a leap day every four years.

Earth's orbit isn't a perfect circle. It's a slightly stretched oval called an ellipse.

Why doesn't the Moon fly off into space or crash into Earth? Two things balance out:

• Inertia: a moving object wants to keep moving in a straight line (Newton's first law).
• Gravity: the pull between objects with mass. The more mass, the stronger the pull. The closer the objects, the stronger the pull.

The Moon is constantly "falling" toward Earth, but it's moving sideways so fast that it keeps missing. That curved path is an orbit. Isaac Newton figured this out, and his law of universal gravitation explains every orbit in the solar system.

In the early 1600s, astronomer Johannes Kepler discovered three rules of planetary motion:

1. Planets orbit the Sun in ellipses, with the Sun at one focus (slightly off-center).
2. Planets move faster when closer to the Sun and slower when farther away. A line from the Sun to a planet sweeps out equal areas in equal times.
3. Planets farther from the Sun take longer to orbit. The math: the square of the orbit time (in years) equals the cube of the average distance (in AU). Mercury zips around in 88 days. Neptune takes 165 years.

Scientists use these laws to predict where planets, comets, and satellites will be, years in advance. That's how NASA lands rovers on Mars.

As you learned in Chapter 22, seasons happen because Earth's axis is tilted about 23.5°, not because of distance from the Sun.

• Summer solstice (around June 21): longest day of the year in the Northern Hemisphere. The Sun is at its highest.
• Winter solstice (around December 21): shortest day.
• Equinoxes (around March 20 and September 22): day and night are about equal everywhere.

The Moon is Earth's only natural satellite, about one-quarter Earth's width and 384,000 km away. It has no air, no liquid water, and a surface covered in craters from impacts. The dark patches, called maria (Latin for "seas"), are actually ancient lava plains. The bright areas are mountainous highlands.

The Moon takes about 27 days to rotate once and about 27 days to orbit Earth. Because those match, the same side always faces us. This is called synchronous rotation. There's a far side we never see from Earth (but it's not always dark).

The Moon doesn't make its own light. It reflects sunlight. Half of the Moon is always lit by the Sun. As the Moon orbits Earth, we see different amounts of that lit half. These are its phases:

1. New moon: the lit side faces away from us. We can't see it.
2. Waxing crescent: a thin sliver on the right. (Waxing means growing.)
3. First quarter: right half lit.
4. Waxing gibbous: more than half lit.
5. Full moon: the whole face is lit.
6. Waning gibbous: shrinking. (Waning means shrinking.)
7. Third (last) quarter: left half lit.
8. Waning crescent: a thin sliver on the left.

The full cycle takes about 29.5 days, which is where the idea of a "month" comes from. Memory trick for the Northern Hemisphere: if it's lit on the right, it's getting bright. If it's lit on the left, it's leaving.

An eclipse happens when one object blocks sunlight from reaching another.

• Solar eclipse: the Moon passes between the Sun and Earth, and its shadow falls on Earth. It can only happen at new moon. In a total solar eclipse, the sky goes dark in the middle of the day. In 2017, the path of totality crossed right over eastern Idaho, including Idaho Falls and Rexburg. Never look at the Sun without proper eclipse glasses.
• Lunar eclipse: Earth passes between the Sun and Moon, and Earth's shadow falls on the Moon. It can only happen at full moon. The Moon often turns a coppery red, called a "blood moon," because Earth's atmosphere bends red sunlight onto it.

The darkest part of a shadow is the umbra; the lighter outer part is the penumbra.

Why don't we get eclipses every month? The Moon's orbit is tilted about 5°, so usually its shadow passes above or below Earth.

As you learned in Chapter 18, the Moon's gravity pulls on Earth's oceans, creating two bulges of water, and Earth rotates through them, causing two high tides and two low tides a day. Spring tides happen at new and full moon; neap tides at the quarter moons.

The Sun is a star, a giant ball of hot gas (mostly hydrogen and helium), about 109 times wider than Earth. It holds 99.8% of the solar system's mass.

In its core, at about 15 million °C, the Sun fuses hydrogen into helium. This process, nuclear fusion, releases enormous energy. (Compare that to fission in nuclear power plants, which splits atoms.) That energy takes thousands of years to work its way out of the Sun, then just 8 minutes and 20 seconds to reach Earth as light.

Sun features:

• Sunspots: cooler, darker areas caused by strong magnetic fields. They come and go in about an 11-year cycle.
• Solar flares: sudden bursts of energy near sunspots.
• Solar wind: a stream of charged particles flowing from the Sun. When it hits Earth's magnetic field, it creates auroras, and big storms can knock out satellites and power grids.