Thursday

Raven spreads a printed table across the Sky Deck. "A NASA fact sheet," she says. "Every planet, in rows."

Rocket runs his finger down a column. "Jupiter's mass is huge. Its gravity number is huge too. Rule one!"

"Keep going," Raven says. "What do you notice about Saturn?"

Rocket frowns. "Saturn has way more mass than Earth. But its gravity number is a bit smaller than Earth's. That breaks the rule."

Nova hovers over the Saturn row. "Does it? Which rule have you forgotten?" she asks.

Rocket looks at the diameter column, then grins. "Distance. Saturn is so wide that its surface is far from its center."

The NASA Planetary Fact Sheet lists numbers for every planet. Today you read three of its rows: mass, diameter and surface gravity.

Mass is given in units of ten to the twenty-fourth kilograms. You only need to compare the numbers, not say them.

Surface gravity is how hard a world pulls on something at its surface. The unit is meters per second squared.

Earth's surface gravity is 9.8. A bigger number means a stronger pull at the surface. A smaller number means a weaker pull.

Rule one shows up first. The Moon has the least mass in this table and the weakest surface gravity. Jupiter has the most of both.

Mars has about one tenth of Earth's mass. Its surface gravity is a little more than a third of Earth's.

Saturn is the puzzle. It has about 95 times Earth's mass, yet its surface gravity is slightly less than Earth's.

Rule two explains it. Gravity weakens with distance from the center. Saturn is so wide that its surface sits very far from its center.

OpenStax Astronomy says your weight depends on the local force of gravity. Your mass stays the same everywhere.

So you would weigh less on Mars and on the Moon than on Earth. You would not have lost any mass at all.

Sharp reading. Tomorrow you follow gravity out of the solar system and into our galaxy.