Constellations

1. What is a constellation? A group of stars that seems to form a picture Constellations

2. What are circumpolar constellations? Constellations that appear to move around the pole star, Polaris They can be seen all year e.g., Ursa Major, Cepheus • What are seasonal constellations? • Constellations that cannot be seen all year. • - e.g. Orion, Leo, Pegasus • What are zodiac • constellations? • Seasonal constellations that are associated with the astrological signs of the zodiac • e.g. Aries, Taurus, Cancer

3. What’s your sign?

4. When we set the star-finders to our birthdays, we discovered we could not see “our” zodiac constellation on the night of our birthday,,,,

5. To understand why, we need to look at Earth in space…. Remember that Earth is tilted on its axis of rotation.

6. So if you look at Earth straight up and down, it would seem like the solar system is tilted instead:

7. The “celestial sphere” That’s why here on Earth, the sun and planets appear to go around the Earth along a slanted pathway that mirrors the plane of the solar system. We call this path the ecliptic.

8. The celestial equator is an imaginary line mirroring Earth’s equator. Notice where the sun is in relation to the celestial equator during the different seasons. What happens on the equinoxes?

9. Here is a view from Earth looking into the sky: The green line is the ecliptic, the pathway the sun appears to follow. The red line is the celestial equator. What day is this? The ancient astrologers chose constellations that are found along the ecliptic as the constellations of the Zodiac.

10. Try to picture the plane of the solar system with the stars and constellations far, far away in all directions. The ones lined up along the plane of the solar system will look like they are on the sun’s path – the ecliptic.

11. Ancient astrologer chose constellations along the ecliptic to represent the signs of the zodiac. Signs are assigned to birthdates according to when the sun appears to be “in” the particular constellation. If the sun is “in” your constellation on your birthday, that means it won’t be out at night, so you will not see “your” zodiac constellation on your birthday!

12. At least it’s supposed to be – but there is a complication! We learned that Earth has 2 major motions: Rotation and Revolution. But there is a THIRD motion……

13. It’s called “precession” and it means that just like a spinning top, Earth wobbles a bit as it spins … A top wobbles because Earth’s gravity is pulling on it.

14. Earth “wobbles” because the moon’s gravity is pulling on it. Unbeknownst to the ancient astrologers, the Earth continually wobbles around its axis in a 25,800-year cycle.

15. That means, Polaris isn’t always our pole star.

16. It also means, that since the time people made up the star signs, the earth has wobbled enough that the sun really isn’t in “your” sign on your birthday. The sun is actually one sign behind – so when the calendar says it’s Aries, the sun is actually in Pisces, and so forth…

17. And here’s another complication, the ancient Babylonians actually had 13 signs! They also included Ophiucus!

18. Which would further change the zodiac sign most people think they have if we decided to bring this sign back: Not sure if whoever made this accounted for precession as well – you can check if you use a Star finder….. (or an iPhone app…)

19. Horoscopes and zodiac signs are fun, but they don’t make any sense scientifically!

20. YOU are the master of your fate, not the stars!!

21. See of you can find the ecliptic and celestial equator on this diagram: