When you think of space, you may think of circles.  Planets shaped like circles.  DING!  Right!  Stars shaped like circles.  DING!  Right!  Earth moving around the sun in circles . . .  Wrong!  Do not worry.  For hundreds of years, even the smartest people thought the planets moved through space in circles.  Space is not as easy as we hoped.  

It's an easy mistake to make.  Just look through a telescope and you will see many circles.  Planets, stars, even the rings around them are in circles.  Circular means to move in a circle or it can mean that something is shaped like a circle.  Up until just a few hundred years ago, everyone believed that the Earth followed this same pattern, moving around the sun like water in a toilet bowl.  Or like a hula hoop.  Wrong.

Hula hoop anyone?

Some things we knew for sure.  First of all, people knew that the Earth turned in place.  It's kind of like spinning a ball on the tip of your finger.  The ball stays in the same place.  It is not going anywhere, but it's still spinning.  Rotation means to spin or turn in one place.  You can try spinning in one place too, but do not get so dizzy you cannot read the rest of this.  The Earth is always spinning in place.  DING!  Right.

While spinning the ball on the tip of your finger, you could also walk around the room.  Now the ball is spinning in place and moving.  The Earth is doing the same thing.  It's rotating while moving around the sun.  To orbit means to move around something else.  Even after we figured out that the Earth was orbiting around the sun, we still believed that it was moving in a circle.  You can make the wrong sound this time.  I do not want to be annoying.

Inner planet orbits are not really circles.

When really smart people looked into the sky, they figured out that the planets could not be moving in circles.  If they were, they would show up at different times and in different places than they were.  Elliptical means to move in an oval shape or to be shaped like an oval.  Think of a running track, or the shape of an egg.  You may also think of a circle drawn by someone who cannot draw circles very well.  The elliptical orbit changes how long each season lasts.  When we orbit closer to the sun, we move faster.  In the northern hemisphere, this makes winter a couple of days shorter than summer.  We are a little closer to the sun during January and this makes the Earth move a little faster.  

Hey, who pulled and stepped on my nice circular orbit?

Fun Fact: We are not alone in the way we move around the sun.  We can see that the other planets move through our sky in the same way.  They also have to move in a squished circle.  Ding!  Right!

It's easy to think that the planets move in circles.  I mean, they are shaped like circles and they spin in circles.  When it comes to orbiting, though, they move around an elliptical orbit.  Try this out!  It may be hard to follow an elliptical path while spinning a ball on your finger, but if you spin in a lot of circles first, it will be impossible for you to walk in a circle.  Ding!  Right!


Windows to the Universe.  "Elliptical Orbits"  Windows to the Universe, 2010.  <http://www.windows2universe.org/physical_science/physics/mechanics/orbit/ellipse.html>

Wise Geek.  "Why Do the Planets Orbit the Sun in an Elliptical Fashion?"  Wise Geek, 2012.  <http://www.wisegeek.org/why-do-the-planets-orbit-the-sun-in-an-elliptical-fashion.htm>