Astronomy 101

02 Earth

Sun-Earth-Moon system

A lot of things are affected by the relationship between earth and the sun and moon. Tides, sunsets, seasons, length of days, the moon. Let’s explore.


The Earth is a sphere. Basically. Like a little squished sphere. It’s a little bigger around the equator than it is at the poles.

Why is this? Because as it’s rotating, it stretches out slightly.

What do terrestrial planets have? Yes. An iron core. An iron core coupled with its rotation gives Earth a magnetic field.

Motion: rotating, revolving

Earth is constantly spinning around an imaginary vertical line running from the South Pole to the North Pole. This motion is called rotation.


One rotation takes 24 hours. Does that length of time sound familiar? Yep. One rotation equals one day.


One revolution around the sun takes 365 days. Technically, 365 and a quarter. A revolution is just a big giant circle. A fancy term might be circumnavigate, but we’ll save that for social studies. So Earth makes a complete circle around the Sun every 365 days which year! The path it takes is called an orbit. Earth’s orbit isn’t an actual circle, it’s more like an ellipse, which means that it’s not the same distance from the sun year-round.


Earth tilts at an angle of 23 degrees from the line that is perpendicular to its orbit. What this means is that light strikes Earth’s surface at different angles when it is at different stages in its orbit.


The combination of orbit and tilt creates seasons. When the northern hemisphere is tilted toward the sun, it gets hit with sun rays at a higher angle and for longer periods of time (long days!). That means it gets more energy, and thus is warmer as well.

The inverse is true: when the northern hemisphere is tilted away from the sun, those rays hit it at a lower angle and for fewer hours. Thus we have winter. Colder temperatures, shorter days.

It’s a zero sum situation. The northern and southern hemispheres are always at opposite seasons. Summer at one, winter at the other. Vice versa. Easy enough. It’s just science.


We call it a solstice when the Earth is most tilted toward the sun. That’s when the sun is its greatest distance north or south of the equator, so it is at its highest or lowest in the sky at noon.

June 21 is the summer solstice and it’s the longest day of the year.

December 21 is the winter solstice and it’s the shortest day of the year.


This is when the Earth is not tilted toward or away from the sun. Happens twice a year. We call it the equinox. On these two days, the length of day is the same all over the world: 12 hours. The sun is directly above the equator.

It happens in the Spring around March 20 and it’s called the Vernal Equinox.

It happens in the Autumn around September 22 and it’s called the Autumnal Equinox.

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