The Earth orbits around the Sun once every 365.26 days. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter every 12 hours. It takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The Earth's rotation is counter-clockwise around it's axis, and the Earth's revolution is counter-clockwise around the Sun. The Earth rotates from West to East, which causes the Sun to rise in the East and set in the West.
Due to complex Newtonian laws of Gravitation, the Earth (like most planets) has a slightly elliptical orbit. The orbital speed of the Earth averages about 30 km/s (108,000 km/h). 30 km/s is fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, or the distance to the Moon (384,000 km) in four hours. That's pretty darned fast. The average distance from the Earth to the Sun is 149,476,000 Km.
It takes 29.5 days for the moon to revolve in it's orbit around the Earth. This is the source of the term "Month" which very roughly corresponds with the phases of the moon. The same side of the moon always faces the earth, because the moon's rotation has slowed down so much. Meanwhile, the moon has a similar slowing effect on the Earth, but slows it's rotation by only 1.5 milliseconds every century.
The Seasons are caused by the Earth's tilt. It's axis is at an angle in regards to the sun, so at any given time one hemisphere receives more direct sunlight than the other. The one getting direct sunlight experiences summer. The other hemisphere gets it's light at an angle, with the photons spread out more obliquely, and so it experiences winter. This angle of sunlight has much more impact on temperature than the minor variations in our orbit. For example, when it's winter in the Northern Hemisphere, we're actually slightly closer to the Sun than when it's summer in the North, but because of the angle, it's colder in winter.
The earth very slightly shifts as it rotates and revolves. This gradual wobble is on a 26,000-year-cycle called precession. The axis points at each sign of the Zodiac in turn during the different stages of that precession. According to Astrology, we are currently in the Age of Pisces, bordering on the Age of Aquarius. The precession also changes which star is the Pole Star. Right now, our magnetic and rotational axis is pointed almost directly at the Pole Star, but 13,000 years into the past or future it would be pointed at Vega.
What if it didn't work like that?
If the earth were sped up so that it rotated more than 800 times in 24 hours, the centrifugal force would be strong enough to overcome gravity, and fling people off the planet. Now that's a disaster movie!
If instead the earth didn't spin on it's axis, and instead kept one face pointed at the sun, then half the globe would be baked, while the other half would be frozen. Perpetual day for one side, and night for the other. This would be disastrous for large lifeforms, who rely on temperature and light cycles. If it happened slowly enough, the earth just reducing it's spin over time, some life would survive. With a faster change, probably only bacteria would survive to live in the twilight equatorial sector.
If the earth's rotation continued, but no longer had it's tilt, that would result in temperature extremes as well. The polar regions would experience constant winter and the equator would have constant summer. Life could survive that, but there'd be no reason to migrate, no such thing as growing seasons, no notion that "love is in the springtime air", no leaves shed in fall, etc.
If the earth didn't orbit the Sun, and instead was a rogue planet wandering the cosmos, it would get extremely cold. Chances are life wouldn't survive that for long. If we still orbited, but from much further out, the days would be dim, and the planet would be mostly cold.
If we were much closer to the Sun, our average temperature would be much warmer, and the sky would be blindingly bright. The Inverse-Square Law tells us that more sunlight would hit every square inch of the planet. So, if our orbit were half it's current radius, we'd get four times as much light and heat as we do now.
Game and Story Use
- These numbers may matter in a hard sci-fi game involving space travel. GMs of such a game should probably also read up on Lagrange points, as those will be useful for describing space stations and stable orbits.
- This information might also prove useful in a time travel game. A mobile time machine, might leave the earth's frame of reference. (See also: Our Time Travel Is Different and Time And Relative Dimensions In Space.) Should that be the case, the machine would need some method of locomotion or a way to anchor itself to a location. 10 hours from now, the earth will be more than a million km from where it is currently, and a person who jumps 10 hours without a frame of reference will find themselves in the vacuum of space. Realistic time travel might require rolls of Science or Math skills to end up at the place (and not just time) you desire, or the time machine itself might handle all that ugly computing for you.
- Perhaps some of the thousands of current missing persons cases are the result of time machine inventors who didn't compensate for the earth's wandering movement.
- Luckily, the Theory of Relativity rejects the notion of absolute time and space, positing that there is no universal truth about the spatial distance between (or temporal simultaneity of) two events. This conveniently makes whatever answer the GM gives the correct one (provided you don't fall into Timey Wimey Ball territory). Time Machines might need conventional engines and have to be sealed against the vacuum of space, or the act of time travel itself might reliably move you to the time and place of your choosing.
- The speculations about changes in our rotation and orbit may prove useful when designing worlds for a science-fiction campaign. See also Random Planetary System, and Alien Biochemistry.
- Two planes leave the same airport at the same time, headed in opposite directions (East and West). They both circle the earth, and return to the airport they left from. They do not, however land at the same time, even though they were flying at the same speed relative to each other. Instead, because of the rotation of the earth, the west-bound plane arrives sooner. The airport (and the Earth itself) has literally moved closer to that plane, and further from the other.