A Sky Full of Stars - I.

! How many stars are visible by eye in the night sky?! Are asterisms and constellations different? How many

constellations are there?! What is the ecliptic? What is the Celestial Equator?

What are Celestial Coordinates and how do they relate to coordinates on the Earth’s surface?

! What are your zenith, horizon, and meridian?! As viewed from the Earth, a star’s position can change

(over timescales of nights or months) for 3 reasons. ! What are the 3 reasons, and how does each reason

cause a star’s position to (quantitatively) change (see also the next lecture)?

Learning Objectives

A Sky Full of Stars! The average person on a clear night can see

about stars!~ total visible (~ half are below the horizon)!All of these stars are in our Galaxy and are

relatively nearby! In 2003, the total number of stars in the known

Universe was estimated to be: !70 sextillion (70 thousand million million million

or 7x1022)!About 10 times the number of grains of sand on

all of the Earth’s beaches and deserts! “Only” ~200 billion (2 x 1011) stars in our Galaxy

The Zodiac! The most famous of the ancient constellations

!Origins deep in our agricultural past...many symbolize planting or harvesting

!12 zodiacal constellations (well, 13 really), one for each lunar cycle (month) per year

! Line traced by the Sun in the sky called the Ecliptic

! If you were watching carefully, the Sun actually passes through 13 constellations, spending ~1 week in Scorpius and ~3 weeks in Ophiuchus

! The Babylonians systemized the zodiac on 12 lunar cycles per year, though, so “standard” astrology only recognizes 12 “star signs”

The Zodiac

Constellation BoundariesAsterisms are star patterns but

constellations have official boundaries, like the borders of a State on a map

There are 88 total constellations

Flattening the Sky! Asterisms are

patterns, as viewed from Earth!Usually not

physically close!Stars in an

asterism can be very far away from each other

! The sky would look very different from another star system

Your View of the Sky! Zenith – the point

directly over yourhead

! Horizon – marks the intersection of the Earth and the sky

! Meridian – the line from North to South through your zenith

3 reasons! If you stay on the surface of the Earth,

there are 3 reasons why stars might move over the course of short time scales (say a few hours to a few years)

! What are they?! Are they East-West motions or North-

South motions?

Celestial Poles and Equator! Celestial poles

! Extensions of the Earth's axis into the sky

! Celestial equator! Project the Earth's

equator into the sky! Celestial sphere

! Extending the Earth’s coordinate system into the sky

North Celestial Pole

South Celestial Pole

Celestial Equator

Celestial Coordinates! North-South

! Earth: Latitude, measured from the equator

! Sky: Declination, measured from the celestial equator

! East-West! Earth: Longitude,

measured from Greenwich, England

! Sky: Right Ascension, measured from the Vernal Equinox (position of the Sun on first day of Spring)

Looking at the Earth from the Celestial Equator

This star visible only from Southern Hemisphere

This star visible only from Northern Hemisphere

star visible for observer near Equator at some point in Earth’s orbit

(some time in the year)

star visible for observer near Equator at some point in Earth’s orbit

(some time in the year)

Important bad drawingLatitude, Zenith, Horizon, Declination

Next Time

A Sky Full of Stars - II.