1. the sky
modern science was born when people tried to understand the sky
(With thanks to Mark Ritter & John Bloom)

2. The stars
first let’s organize the sky

3. Constellations
In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures.

4. constellations
the ones we know started in ancient Mesopotamia, then went to Babylon, Greece, Rome…

5. Constellations
Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

6. there are 88 of them
there are also things called asterisms; groups of stars that usually look like something
like the Big Dipper, and the Great Square of Pegasus
most constellations and asterisms are made of stars that are not physically associated with each other…

7. The stars of a constellation only appear to be close to one another
Usually, this is only a projection effect.
The stars of a constellation may be located at very different distances from us.

8. the names of stars Arabic
most constellations are in Latin, but most stars derive their names from…
Arabic
e.g. Betelgeuse came from yad al-jawza, the giant’s armpit

9.  names don’t say much and we run out of them soon
another way is to name them with greek letters from a on (alpha usually is brightest, beta next, and so on)
so it is Greek letter + NAME
e.g.:
 Centauri, b Gemini, g Canis Majoris

10. The Magnitude Scale First introduced by Hipparchus (160 - 127 B.C.):
Brightest stars: ~1st magnitude
Faintest stars (unaided eye): 6th magnitude
More quantitative:
1st mag. stars appear 100 times brighter than 6th mag. stars
1 mag. difference gives a factor of in apparent brightness (larger magnitude => fainter object!)

11. The Magnitude Scale
The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects):
Sirius (brightest star in the sky): mv = Full moon: mv = Sun: mv = -26.5

12. moreover, all these are just how they look at visible wavelengths
(apparent visual magnitude mv is another name for it)
but what about those that pour out uv or x-rays?
and what about eyes that are more sensitive than yours?

13. the sky and its motion
we have to go back into the past to get a feel for the sky now…

14. the celestial sphere
ancient astronomers believed the sky was a great dome, with stars stuck on it
of course, it’s not, but it’s still convenient to see it that way

15. why did people make this model of the cosmos?
because it looks like it!
the whole sky like a giant sphere seems to travel around us
ready for some vocabulary?

16. zenith & horizon

20. angular distance is measured in degrees
1/60 of a degree is an arc minute
1/60 of an arc minute is an arc second
here are simple ways to measure

21. circumpolar constellations are ones that never go below the horizon
at NP, all are cp,
at equator , there are none…
we have a few

22. what we see depends on where we are
see that the angle the NCP is above horizon is our latitude

23. precession
Hipparchus first noticed that the NCP wasn’t fixed; it was slowly moving!
Earth spins around like a top
Its wobble is called precession
takes 26,000 years!

24. this is how the sky was when Thuban was North Star (~3000 BC)
the Sun & Moon pull on our wider equatorial region so we don’t wobble over
it’s the perfect wobble!