Orbit and Tilt – Are You Dizzy Yet?

I admit it. For a long time during my enthusiasm for astronomy, I had a hard time wrapping my head around terms such as the ecliptic.

The September issue of Sky & Telescope magazine re-boots a column originally named Beginners Page. It started with the inaugural issue, only to fade away. The column, renamed as Beginner’s Space, is exactly that, a column for beginners, or folks like me, who find certain concepts challenging.

Their first column… What is the Ecliptic? I am grateful and I plan to exploit Sky & Telescope even more.

And shame on me, I didn’t explain the ecliptic in my previous articles about astronomy terms. If I had, it would be this: It’s the plane of Earth’s orbit around the Sun.

So, we go around the Sun once per year and if someone were to capture it and speed the video up, the orbit would look like an elongated circle. That elongated circle is the ecliptic.

Every planet has an ecliptic – it’s their orbit plane around the Sun. Guess what, solar system planets pretty much go around the Sun in a similar orbital plane as ours. Why? Astronomers see this as evidence of our solar system beginning as a disk of debris around a young star, we now call the Sun. Most planets orbital planes are a little offset from the others, with only two planets in the same orbital plane, Mars and Neptune.

Astronomers project this orbital plane outward on our sky and what do they find? From our point of view, the planets and the Sun follow this curved line. No matter what time of year, the Sun and planets will be on or near this line in the sky. Certain constellations also follow the ecliptic – constellations of the zodiac.

In our solar system, in our Milky Way galaxy, in our universe, most things are spinning on an axis while orbiting around something else.

Spinning on an axis, like Earth, results in other phenomena – poles and an equator. The poles are like the points where a rod would go through with the Earth spinning on it. The equator is the mid-point on Earth between poles. If the Earth’s orbital plane and its spinning on axis are the same, the equator and ecliptic would be the same – yea!  The Sun and planets would march across the sky in line with the horizon.  But no, they are not. Our Earth’s rotation axis is tilted compared to our orbital plane. The tilt is 23.4 degrees, so Earth’s rotation is tilted 23.4 degrees from its orbit. That complicates the picture. The Sun, planets, stars, everything appears to rise and move at an angle across the sky.

It’s this tilt that contributes to our varying seasons too, for most of the planet.

What’s in the Sky?

August 12/13; 9pm on the 12th until the following dawn; northeast: The Perseid meteor shower peaks but a bright Moon interferes.