# Astro 1 - Lecture 2

## Penn State University

Fall 1996

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 Lectures Lecture page Astro1 page

# THE CELESTIAL SPHERE

## 1. The celestial sphere

celestial equator

celestial poles (N and S)

celestial coordinates

RA - right ascension (like longitude)

hours (24), minutes (60), seconds (60) of time

DEC - declination (like latitude)

degrees (360), arcminutes (60), and arcseconds (60) of arc

## 2. Our view

a) Earth's rotation on its axis

- this axis defines the North and South celestial poles

b) Earth's orbit around the Sun

These two motions define the ``visible'' constellations

what we see at night!

## 3. Diurnal motion of the Sun on the celestial sphere

Again: the Earth orbits around the Sun

This orbit is in a plane defined as the ecliptic

From our perspective (looking out from the Earth):

The ecliptic is also the path of the sun on the
celestial sphere during the course of a year

But!

The Earth's spin axis is tilted with respect to the orbital plane.

By how much?
23.5 degrees

As a result:
seasons -
winter and summer solstice
vernal and autumnal equinox

## 4. Sidereal and solar days

What's the difference? minutes! (solar is longer)

Why:

360 degress in a circle

365 days in a year

Earth moves about 1 degree per day in its nearly circular orbit around the Sun

This means the Earth has to spin an extra degree per day until the Sun is ``straight overhead'' some given part of the Earth ... so ...

24 hours x 60 minutes per hour x = 4 minutes

Q2.1 How is the observable universe like a time machine?

(a) the universe expands as it ages

(b) there was a beginning of time, and time moves in only one direction

(c) the speed of light is finite, the hence light from more distant objects takes longer to reach us

(d) bigger objects are older

(e) the speed of light is increases in the past

Q2.2 What best describes the ecliptic?

(a) the celestial equator

(b) the plane of the Sun's orbit around the Earth

(c) the Earth's tilt with respect to the fixed stars

(d) the line connecting the N and S celestial poles

(e) the sun's path on the celestial sphere

 Lectures Lecture page Astro1 page