The sky modern science was born when people tried to understand the sky (With thanks to Mark Ritter...

the sky

modern science was born when people tried to understand the sky

(With thanks to Mark Ritter & John Bloom)

The stars

• first let’s organizethe sky

Constellations

In ancient times, constellations only referred to the brightest stars that appeared to form

groups, representing mythological figures.

constellations

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

Constellations

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

• 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…

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.

the names of stars

• 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

• 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

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 6 th mag. stars

• 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude => fainter object!)

The Magnitude Scale

Sirius (brightest star in the sky): mv = -1.42Full moon: mv = -12.5

Sun: mv = -26.5

The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects):

• 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 sensitivethan yours?

the skyand its motion

• we have to go back into the past to get a feel for the sky now…

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

• 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?

• zenith & horizon

• 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

• circumpolar constellations are ones that never go below the horizon

• at NP, all are cp,• at equator , there

are none…• we have a few

• what we see depends on where we are

• see that the angle the NCP is above horizon is ourlatitude

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!

• 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!