Tuesday

Raven holds two hoops, one inside the other, and tilts the smaller one a little. "Earth's orbit. The Moon's orbit."

Rocket stares. "They only cross in two places."

"What do you notice about every other spot?" Raven asks. "The Moon is above or below," Rocket says. "So its shadow misses."

Nova hovers over the hoops. "About 5 degrees of tilt," she says. "Enough to miss most months."

"And when it lines up?" Rocket asks. "Eclipse season," Raven says. "Twice a year."

Rocket grabs the orange and the big ball. "Then the Moon has to be exactly the right size to cover the Sun."

Nova hums. "Lucky, is it not? Read why."

Suppose the Moon's path in the sky matched the Sun's path. Then we might see an eclipse of the Sun and the Moon each month.

But the Moon's orbit is tilted about 5 degrees relative to the plane of Earth's orbit around the Sun.

OpenStax says to imagine two hula hoops with a common center, tilted a bit. The hoops cross in only two places.

During most months the Moon is far enough above or below the plane to avoid an eclipse. Its shadow misses Earth, and Earth's shadow misses it.

When the two paths cross, twice a year, it is eclipse season, and eclipses are possible.

NASA says Earth, Moon and Sun line up for an eclipse anywhere from four to seven times a year.

The Sun is about 400 times larger in diameter than the Moon. It is also about 400 times farther away.

So both the Sun and the Moon appear about half a degree wide in our sky. That is why the Moon can just cover the Sun.

OpenStax calls this a coincidence of living on Earth at the present time. NASA calls it a lucky chance of nature.

When the Moon is a little farther from Earth, it looks slightly smaller than the Sun. Then a ring of Sun shows around it.

Clear thinking. Tomorrow is Sky Deck Lab: you make both eclipses with the lamp, and then make them miss.