constellations constellations -- groups of stars named by ancient cultures to honor gods, animals,...
TRANSCRIPT
Constellations
Constellations -- groups of stars named by ancient cultures to honor gods, animals, legends, etc.
They provide us with ways to recognize and identify individual stars and groups of stars (i.e., they are a crude “map” of the sky).
In all, there are 88 constellations; 12 in the zodiac (13 really).
To learn more about constellations see:
The Stars by H.A. Rey
http://StarrySkies.com/
Sky & Telescope and Astronomy magazines
How Many Constellations?
Celestial Sphere
The celestial sphere:
Stars seem to be on the inner surface of a sphere surrounding the Earth
They aren’t, but can use two-dimensional spherical coordinates (similar to latitude and longitude) to locate sky objects
Eastern Sky in the Winter
Orion is a primary Winter constellation
Eastern Sky in the Summer
The Summer Triangle
Constellations, contStars that appear close in the sky may not actually be close in space:
Big/Little Dippers
From our latitude on the Earth, Polaris (the North Star) and the stars around it can be seen all night long.
The stars appear to rotate around Polaris.
Constellation Boundaries
A constellation is not just the stars that make up a “picture”, it is an area of sky that borders the surrounding constellations.
Celestial Sphere
The celestial sphere:
Stars seem to be on the inner surface of a sphere surrounding the Earth
They aren’t, but can use two-dimensional spherical coordinates (similar to latitude and longitude) to locate sky objects
Right Ascension and Declination• Declination: degrees north or south of celestial equator• Right ascension: measured in hours, minutes, and seconds eastward from position of Sun at vernal equinox
The Celestial SphereEquator => Celestial Equator
North Pole => North Celestial Pole
South Pole => South Celestial Pole
Celestial Sphere is imaginary, not real. It is used for convenience.
What happens when you need to know where an object is more accurately than “the fourth brightest star in Taurus”?
longitude
latitude
Right Ascension (RA)
Declination (Dec)
Define a new coordinate system: the Celestial Sphere
The celestial sphere is essentially a projection of Earth’s coordinate system of longitude and latitude out into space.
So, instead of saying “The object is just to the left of Rigel in the constellation of Orion”, we have coordinates much like longitude and latitude on the Earth.
SkyGazer Demo
Rotation of the Earth
Earth’s rotation around its axis:
• Causes night and day (Solar day = average time between consecutive “noontimes”).
• Causes (apparent) motion of the stars
Years
Time for Earth to orbit once around Sun, relative to fixed stars, is sidereal year
Tropical year follows seasons; sidereal year follows constellations – in 13,000 years July and August will still be summer, but Orion will be a summer constellation
Daily/Nightly Motion of the Stars
The rotation of the Earth causes the stars in the night sky to rotate.
Depending on where you are on the Earth, the pattern of rotation is different.
Stars and constellations that never set (as seen from your location) are called circumpolar (they rotate around the pole).
Circumpolar stars rotate around Polaris (in the Northern Hemisphere).
At the North and South Pole, the stars move horizontally.
On the Equator, the stars move vertically.
At mid-latitudes, the stars move “diagonally” on an arc across the sky.
Position tutorial page 1-2
Daily Motion
Yearly Motion of the StarsSo far we have just talked about the motion of the stars over the course of a night (due to the Earth’s rotation).
Over a day, the Sun will not appear to move with respect to the constellations.
But over the course of months or a year, the Sun appears to move.
Yearly Motion of the Stars12 constellations Sun moves through during the year are called the zodiac; path is the ecliptic
Yearly Motion
Ecliptic tutorial page 13-17 (I-III, IV opt)Seasonal Stars tutorial page 7-9 (take home if don’t finish), maybe do this after Horoscopes depending on time.
(Fall 2008 – completely ran out of time!)
Horoscopes
Read your horoscopes. Which one sounds most like you?
a) Your signb) The previous signc) The next signd) Six signs later
Precession – Why Your Sign is WrongPrecession: rotation of Earth’s axis itself; makes
one complete circle in about 26,000 years
Years: Two Types
Time for Earth to orbit once around Sun, relative to fixed stars, is a sidereal year
A tropical year is the time it takes for the Earth to be in the same configuration with respect to the Sun again (as opposed to distant stars).
Tropical year follows seasons; sidereal year follows constellations – in 13,000 years July and August will still be summer, but the “Summer Triangle” will be a winter constellation.
Next Time
Seasons and Lunar Phases
Reading: E2, E3 5.2 (12 pages) HW #1 Due