1. The Sky

2. Essential Questions How do astronomers refer to stars?
How can you compare the brightness of the stars?
How does the sky move as Earth rotates?
What causes the seasons?

3. Constellations
Constellations are groups of stars that appear relatively close together and which are said to represent people, gods, and objects.

4. What we see…

5. Asterisms
Many groupings we recognize in the sky are only parts of constellations or parts of multiple constellations. Such groupings are called asterisms.
Examples include: The Big Dipper in Ursa Major, the Great Square of Pegasus in Pegasus and Andromeda, and the Summer Triangle in Lyra, Aquila, and Cygnus.

6. Stellar Nomenclature (star naming)
Names of constellations are in Latin.
Many bright stars also have names and most names derive from Arabic. For example: Betelgeuse, the bright red star in Orion, comes from the Arabic yad al-jawza, meaning “armpit of Jawza [Orion].”
We’ll learn some more star names later, but astronomers have another way to identify stars…

7. Start with brightest and go Greek
Another way to identify stars is to assign Greek letters to the bright stars in a constellation in approximate order of brightness.
The brightest star would be designated by alpha (α) and the next by beta (β), then gamma (γ), delta (δ), etc.
Now an example:

8. Nomenclature Example
The brightest star in the constellation Canis Major (The Great Dog) would be called
α Canis Majoris
This star happens to be the brightest star in the night sky and is more commonly known by its Arabic name—
Sirius
That reminds me of something…

9. Brightness of Stars
The brightness of stars is measured on the magnitude scale.
Invented by Greek astronomer Hipparchus.
Ancients divided stars in six divisions.
The brightest stars were of 1st magnitude.
The dimmest stars were of 6th magnitude.
Modern astronomers have extended the scale to include fainter and brighter objects.

10. Magnitude Scale Bright objects have a small or negative magnitude.
Dim objects have a large and positive magnitude.
Kind of backwards isn’t it?
2.5 times for each difference
Apparent Visual Magnitude

11. A Model of the Sky
Ancient astronomers believed the sky was a great sphere surrounding Earth with the stars stuck on the inside like thumbtacks in a ceiling.
This Celestial Sphere model is not true, but can you see why the ancients believed it? Have you observed the motions of objects in the sky?

12. Celestial Sphere At the north celestial pole is a star…
Polaris (North Star) in the constellation Ursa Minor (Little Dipper)
There is no bright star directly at the south celestial pole.

13. Some definitions:
Zenith—the point on the sky directly above your head.
Horizon—the circle on the sky where it meets the ground.
Celestial Equator—the circle on the “celestial sphere” halfway between north and south celestial poles.
Ecliptic—the path of the Sun in the sky.

14. Circumpolar Constellations
Circumpolar constellations are those that circle the pole star, Polaris.
They are always visible from Ohio. Just look north!
Constellations in the south are different…more on that later.

15. Precession
The Earth spins like a spinning top tipped at a 23.5° angle.
As tops do sometimes, the Earth wobbles and its north pole points at different places. This is called precession.
An entire wobble takes about 26,000 years!

16. The Zodiac

17. Zodiacal Constellations
Do you see why the zodiacal constellations are only visible during certain seasons?
I was born in June and my “zodiacal sign” is Gemini.
When is the constellation Gemini visible in the sky?
What did that mean about the sign under which I was born? Where was Gemini?

18. The Seasons
Many people believe we have cool winters and warm summers because the Earth gets closer and farther away from the Sun over the year.
While the Earth’s orbit is elliptical, and we do get about 1.7% closer/farther to the Sun than average, this is not the cause of the seasons.

19. Perihelion & Aphelion
In fact, the Earth is at perihelion, its closest point to the Sun, around January 4th when we have winter. (~147,500,000 km)
The Earth is at aphelion, its farthest point from the Sun, around July 4th during our summer. (~152,500,000km)

20. What causes the seasons then?
The tilt of the Earth’s axis.
Throughout the year, the Earth’s North Pole always points towards Polaris.
Sunlight hits the Earth at different angles in the four seasons.

21. The Seasons
In the summer, the Sun is above the horizon longer because it rises north of east and sets north of west. (see diagram)
It also shines more directly down on the ground. (think of flashlight shining straight down or at an angle)
Both effects cause warmer weather.

22. Solstices
The Sun is highest in the sky on the Summer Solstice, June 21st or so. On this day, the Sun rises the farthest N of E and there are the most daylight hours.
The Sun is lowest in the sky on the Winter Solstice, December 21st or so. On this day, the Sun rises the farthest S of E and there are the least daylight hours.

23. Equinoxes
On March 21st or so, the Vernal (Spring) Equinox occurs. The Sun rises due East and we have equal hours of daylight and night.
On September 21st or so, the Autumnal (Fall) Equinox occurs. The Sun again rises due East and we have equal hours of day and night.

24. Direct Sunlight on Solstices and Equinoxes
On the Equinoxes, the Sun’s rays shine straight down on the Earth’s Equator.
On the Summer Solstice, the Sun shines straight down on the ‘Tropic of Cancer’ (23.5° North).
On the Winter Solstice, the Sun shines straight down on the ‘Tropic of Capricorn’ (23.5° South).