ASTRO 101 Principles of Astronomy

Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: Telephone: WWW: Office: Physics 241, hours T TH 3:30-5:00

Text: “Discovering the Essential Universe, Fifth Edition” by Neil F. Comins

Course WWW Page Note the underline: … ast101_spring2013a.html … Also check out Nick Strobel’s Astronomy Notes:

Homework due January 29: Question 15 from Chapter 1 (Why is it warmer in the summer than in winter?) Write down the answer on a sheet of paper and hand it in before the end of class on January 29.

Speaking of infinity… Which set is larger: The set of positive integers (1,2,3,…) The set of even integers (2,4,6,…)

Speaking of infinity… Which set is larger: The set of positive integers (1,2,3,…) The set of even integers (2,4,6,…) Both are the same size since there is a one- to-one mapping between the two: (1, 2, 3, 4, …) (2, 4, 6, 8, …)

Pop Quiz What is the shape of the Earth?

The Earth How do we know the Earth is spherical?

The Earth How do we know the Earth is spherical? –Curved shadow of the Earth on Moon during eclipse –The manner in which ships at sea disappear when they sail away –The fact that as you go north-south certain stars disappear

The Earth How do we know the Earth is spherical? –Curved shadow of the Earth on Moon during eclipse –The manner in which ships at sea disappear when they sail away –The fact that as you go north-south certain stars disappear

Questions for Today What causes the seasons? Let’s ask some Harvard students: Does the Earth go around the Sun? Let’s ask…

Next: Discovering the Night Sky

Coming Up: Introduction to the Sky –Constellations –Stellar Brightness –Stellar coordinates and the Celestial Sphere –The “clockwork” of the sky Day/night Phases of the moon The seasons

Looking at the Night Sky On any given night, you can see about 3000 stars without a telescope, provided the sky is dark.

Constellations People have long made up stories about groups of stars that appear close together on the sky. Such groupings are called constellations. The sky was “officially” divided up into 88 constellations in 1930 so that a star is associated with only one constellation.

Constellations The modern constellations have strictly defined boundaries by international agreement.

Constellations Many constellation names are derived from characters in Greek or Roman mythology. Here is Cassiopeia, with its distinctive “W”. The stars are usually not physically associated with each other.

Constellations Here is the “Big Dipper”, which is not an “official” constellation but part of a larger one. Again, the stars are usually not physically associated with each other.

Constellations Constellations can help you mark the seasons.

Stellar Brightness: Magnitudes Historically (e.g. Hipparcos in the First Century), the brightness of stars as seen by the eye have been measured on a magnitude scale: –The brightest stars were “first magnitude”. –The faintest stars were “sixth magnitude”. Brighter objects have smaller magnitudes.

Stellar Brightness: Magnitudes In modern times, it was discovered that the human eye has a nonlinear response to light: if one source of light has twice the light as a second source, then the first source would not appear by eye to be twice as bright. The response of the eye is logarithmic, so that differences of magnitudes correspond to ratios of brightness.

The Magnitude Scale The modern of the magnitude scale is set up so that a difference of 5 magnitudes corresponds to a ratio of brightnesses of 100. Bright objects can have negative apparent magnitudes.

The Celestial Sphere Imagine the sky as a hollow sphere with the stars attached to it. This sphere rotates once every 24 hours. This imaginary sphere is called the celestial sphere. Even though we know it is not the case, it is useful to imagine the Earth as being stationary while the celestial sphere rotates around it.

The Celestial Sphere The north celestial pole is directly above the north pole on the Earth. The south celestial pole is directly above the south pole on the Earth. The celestial equator is an extension of the Earth’s equator on the sky. The zenith is the point directly over your head. The horizon is the circle 90 degrees from the zenith.

The Celestial Sphere The celestial poles and the celestial equator are the same for everyone. The zenith and the horizon depend on where you stand.

Stellar Coordinates and Precession There are a few ways to specify the location of a star (or planet) on the sky: Altitude/Azimuth: –The altitude describes how many degrees the star is above the horizon, the azimuth describes how far the star is in the east-west direction from north. –The altitude and azimuth of a star is constantly changing owing to the motion of the star on the sky!

Stellar Coordinates and Precession There are a few ways to specify the location of a star (or planet) on the sky: Equatorial system: –Lines of longitude on the earth become right ascension, measured in units of time. The RA increases in the easterly direction. –Lines on latitude on the earth become declination, measured in units of degrees. DEC=90 o at the north celestial pole, 0 o at the equator, and -90 o at the south celestial pole. –

Stellar Coordinates and Precession The north celestial pole moves with respect to the stars very slowly with time, taking 26,000 years to complete one full circle.