This is what it’s all about…

The Celestial Sphere Useful concept for: 1.Finding your way around the sky (astronomical coordinate system) 2.Thinking about the motions of the earth

Your local “zenith” and “meridian” Great circle that connects the NCP, the SCP, and zenith is the “meridian” Locally, your meridian separates “East” from “West”

Coordinates on the Celestial Sphere Declination (  ): analogous to latitude on the Earth (celestial equator defines the “zero” of declination) Right Ascension (  ): analogous to longitude on the Earth (location of sun on first day of spring defines the “zero” of right ascension)

The Celestial Sphere Regions of the celestial sphere are divided up into “Constellations” Only the very brightest stars in a handful of constellations are indicated here

Constellations - 88 in total Some constellations are easy to recognize from the patterns of stars. Very few are easy to imagine as their corresponding image (mythological character or animal). Orion (the hunter) is one of the exceptions!

Cassiopeia - The Queen (mother to Andromeda); see Clash of the Titans (they’re all in the sky) Constellations have well-defined borders on the sky (set by the International Astronomical Union in 1928)

Constellations of the Zodiac (the “sun sign” constellations of astrology) Called the sun signs because they are all located along “the ecliptic”, the path of the sun through the sky (= the earth’s orbit around the sun). Over the course of a year, the sun (as viewed from the earth), “travels” through each of these constellations.

Where (approximately) was this picture was taken?

The Celestial Sphere Which constellations you can see on any given night depends on 1.Your latitude (you can’t see through the earth!) 2.The time of year (or “season”)

With the exception of stars that are near the NCP, astronomical objects “rise” & “set” when seen from Boston

Constellations of the Zodiac (the “sun sign” constellations of astrology) Called the sun signs because they are all located along “the ecliptic”, the apparent path of the sun through the sky (= the earth’s orbit around the sun). Over the course of a year, the sun (as viewed from the earth), the sun travels through each of these constellations.

Zodiacal Constellations and the Sun Which constellations could you SEE (i.e., on a given night)? Hint: if you can see the sun, you can’t see any other stars

Path of the Sun at Different Times of Year: Northern Hemipshere From March (spring equinox) to September (autumnal equinox), the sun rises and sets north of due east/due west, so the days are long. From September to March 21-22, the sun rises and sets south of due east/due west, so the days are short. Equniox = equal night The height of the sun at noon and its rising/setting point depend upon your latitude on earth. The poles are truly extreme (constant day/night for months).

Midnight Sun Near the earth’s poles, the sun never sets during mid-summer. You pay for all this summer sun by never seeing the sun in mid-winter!

Tilt of the Earth Relative to the Sun’s Rays The earth’s equator is inclined at 23.5 o to the plane of its orbit around the sun (the “ecliptic”). This is the true cause of the seasons. What season is it in South America? What season is it in North America?

Solar “Irradiance” The greater is the angle between the surface of the earth and the sun’s rays, the more power per unit area the surface of the earth receives.

Seasons and the Earth’s Orbit You experience winter when your hemisphere is pointed away from the direction of the sun, and summer when your hemisphere is pointed toward the direction of the sun.

Precession of the Equinoxes Like a spinning top, the earth’s rotation axis “precesses”, constantly changing the direction of the North pole with respect to the sky. This “minor motion” is very slow (takes 26,000 years to complete), but is important to navigation by the stars and causes seasons to shift! Right now, the North Star is “Polaris” (the tail star of the Little Dipper). Five thousand years ago the North Star was Thuban, and in 14,000 it will be Vega.