Foundations of Astronomy The Metric System (used by scientists and foreigners) Mass 1 kilogram (kg) = 1000 grams (g) 100 kg = 220 lbs We tend to use mass and weight interchangeably, but weight depends on gravity. Distance 1 meter (m) = 100 centimeters (cm) 1 m = 1.1 yards (approx.) 1 cm = 0.4 inches (approx.) Volume 1 cubic centimeter or 1 cm 3 (about the size of a sugar cube)

Density Density = Mass (g / cm 3 ) Volume Densities of Common Substances Balsa Wood 0.13 g / cm 3 Oak 0.7 Water 1.0 Average Rock 2.4 Iron 7.9 Lead 11.3 Gold 19.3

Temperature The Celsius Scale: 0 o C = freezing point of water ( = 32 F) 100 o C = boiling point of water ( = 212 F) The Kelvin Scale: T(K) = T( o C) o C "Absolute zero" 0 K = o C

Angles 90 o 20 o 360 o, or 360 degrees, in a circle. 1 o = 60' or arcminutes 1' = 60" or arcseconds 1" = 1000 mas or milli-arcseconds

Scientific Notation Powers, or exponents, of 10: 100 = 10 2 (= 10 x 10) 1000 = 10 3 (= 10 x 10 x 10) 10 = = = = 7 x x 0.002= (4 x 10 3 ) x (2 x ) = 8 x 10 0 = 8 Add the exponents

In astronomy, we deal with: 1. Vast distances - Radius of Earth = 6400 km = 6.4 x 10 8 cm - Distance to Sun = 1.5 x cm = Earth radii = 1 Astronomical Unit (AU) - Distance to next nearest star (Proxima Centauri): 270,000 AU = 4.3 "light years" (light year: distance light travels in one year, 9.5 x km. Speed of light: c = 3 x 10 8 m/sec) - Size of Milky Way Galaxy: about 100,000 light years - Distance to next large galaxy (Andromeda): 2 x 10 6 light years

2. Huge masses: - Mass of Earth = 6 x kg = 6 x g = 1 M Earth (or 6000 billion billion tons) - Mass of Sun = 2 x kg = 2 x g = 1 M Sun = 1 "Solar Mass" = 333,000 M Earth - Mass of Milky Way galaxy: 6 x M Sun

3. Long ages and times: - Age of Earth and Solar System: 4.5 billion years = 4.5 x 10 9 years - Lifetime of stars: about years - Age of universe: about years 4. Very high and low temperatures: - An interstellar "molecular cloud": T = 10 K - Center of Sun: T = 1.5 x 10 7 K

The Sky at Night What do we see? The Moon Planets Perhaps a meteor shower, comet, or other rare event Stars - about 3000 visible Patterns of stars - constellations 88 of them Useful for finding our way around the sky, navigating the oceans Satellites, airplanes, clouds, lightning, light pollution...

The Celestial Sphere Features: - Does not rotate with Earth -Poles -- aligned with Earth’s North Pole and South Pole -- so, called the “North Celestial Pole” and “South Celetial Pole” -- Equator -- aligned with the Earth’s equator (in the same plane) -- referred to as the “Celestial Equator” -Coordinate System -- Right ascension: parallel to lines of longitude, measured in units of time -- Declination: parallel to lines of latitude, measured in degrees like latitude An ancient concept, as if all objects at same distance. But to find things on sky, don't need to know their distance, so still useful today. < -- a long-exposure photo shows star trails The arcs are along each star’s declination

The "Solar Day" and the "Sidereal Day" Solar Day How long it takes for the Sun to return to the same position in the sky (24 hours). Sidereal Day How long it takes for the Earth to rotate 360 o on its axis. These are not the same!

One solar day later, the Earth has rotated slightly more than 360 o. A solar day is longer than a sidereal day by 3.9 minutes (24 hours vs. 23 hours 56 minutes seconds) deg Earth’s orbit Day 1 Day 2 Solar vs. Sidereal Day : Earth’s motion along its orbit, combined with its rotation about its axis, causes the lengths of these days to be slightly different. Solar day = noon to noon (Sun at same position in sky) Sidereal day = time for a celestial object to rotate back to same position in sky

Ancient cultures observed the sky Egypt...

... ancient Britons native Americans...