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

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Presentation transcript:

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 organize the 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 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

 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 6th mag. stars 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude => fainter object!)

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 = -1.42 Full moon: mv = -12.5 Sun: mv = -26.5

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?

the sky and 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 our latitude

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!