2 Chapter 2 – Why don’t we always see the same stars?
OpenStax Astronomy Chapter 2 Section 1: The Sky Above
The Celestial Sphere
I. Earth rotates on its axis.
A. As viewed from above the North Pole
B. One rotation =
C. Objects in sky “move”
The Sun rises in the East and sets in the West. Which way is Earth rotating?
A. Toward the West
B. Toward the North
C. Toward the East
D. Toward the South
Earth-centered viewpoint
II. Sphere
A. Projection of Earth’s and into space
B. Earth at
C. ! Stars are actually at different distances, and Earth is not in center
1. Like a
D. Appears to rotate around the each day (every 24 hrs).
1. North celestial pole is the (Polaris)
2. Remember it is really EARTH that rotates!
E. : midway between poles, extension of Earth’s equator.
Ecliptic and Constellations
III. : imaginary line on Celestial Sphere
A. Yearly path of the on the Celestial Sphere
B. by 23.5 degrees to celestial equator
1. Because is tilted by same amount to its orbit
C. The constellations are located on the Ecliptic
1. Zodiac: Thirteen constellations that the appears to pass through each
D. of the Solar System – planet orbits all lie on same plane
E. : Group of stars that form a picture on the sky (like the Big Dipper)
1. Stars are physically to one another
2. Astronomers use them to indicate on the Celestial Sphere
What Stars We See
IV. What stars can we see: at the on Earth
A. See of the Celestial Sphere.
1. South Celestial Hemisphere below horizon.
B. The North Celestial Pole – – is directly
1. at the
C. No star rises or sets: all are
1. and circle the celestial pole
D. Ecliptic is low or – no in winter
1. Observers can never see the Celestial Pole
2. At Pole: same rules but see different stars (South Celestial Hemisphere)
If you were standing on the Earth’s South Pole, which stars would you see rising and setting?
A. All of them
B. None of them
C. Some of them
V. At Earth’s
A. All stars
1. None are circumpolar
B. Celestial Poles are on the
C. See the Celestial Sphere as it rotates
VI. At (where we are)
A. Some stars are
B. Some stars
C. Some stars are visible
D. Celestial Pole is above the horizon
1. North Pole in Northern hemisphere
E. Ecliptic is in sky and visible
F. in sky of celestial pole =
on Earth
1. Altitude = the angle above the horizon
2. North Celestial Pole is at 90 degrees (zenith) if you are at the North Pole
At a latitude of +50°, how far above the horizon is the North Celestial Pole?
A. 0°
B. 40°
C. 50°
D. 90°
If you were standing on Earth’s North Pole, where would the North Star be?
A. Straight overhead
B. On the horizon
C. At an altitude of 30° N
Earth’s Orbit and Axis
Change perspective to SUN-centered viewpoint
VII. Earth’s (or revolution) around the Sun
A. Almost
B. Distance to the Sun from Earth = 1 (AU)
1. 1 AU = 150 million kilometers. This is an distance
2. The Earth is closer to the Sun in (147.5 million km)
C. Defines the Plane and the line on Celestial Sphere
1. All orbit the Sun on the Ecliptic Plane
D. The Sun’s motion on the Ecliptic (Earth-centered view) is because of Earth’s around the Sun.
E. As Earth orbits, the Sun is seen different constellations that lie on the Ecliptic
1. These constellations are called the
2. Can’t see the constellation Sun is “in” at any given time because it is
VIII. Earth’s
A. perpendicular (90 degrees) to the Ecliptic Plane
B. Always at an angle of degrees
1. The Ecliptic is tilted 23.5 degrees to the Celestial Equator
C. Sun appears either of (above or below) Celestial Equator over a year
1. The Sun’s path on the Celestial Sphere is on the Ecliptic not the Equator
D. This is why there are , light from Sun not same all year
Resources
NAAP Rotating Sky Simulation: https://astro.unl.edu/naap/motion2/animations/ce_hc.html
Tutorial Activity – Motions of Stars
“Elementary Astronomy Worksheet Handout 3: Motions of Stars, Seasons” (modified by Kaisa E. Young) by Catherine Whiting via OER Commons, licensed under CC BY 4.0, https://oercommons.org/courses/elementary-astronomy-worksheets
1. Fill in the blanks: The Earth once per day and the Sun once per year.
2. What is at your Zenith right now?
Is your Zenith different from someone else in the class?
3. What is the ecliptic?
Why does it not line up with the celestial equator?
4. Why do the stars appear to rise and set in the sky?
(a) Do all of the stars you see from home in a given night rise and set? What are stars that don’t set called?
(b) If you were on the north pole, how would the stars appear to move across the sky? Where would
the North star be located?
(c) If you were on the equator, how would the stars appear to move across the sky? Where would the North star be?
5. Thibodaux, LA is at 29.8° North latitude.
(a) Where should you look (specify direction and altitude above your horizon) in the sky to find the North star?
(b) If you were to drive to Little Rock, Arkansas, which is north of Thibodaux, how would
the altitude of the north celestial pole and the North star change in the sky? Will it appear
higher or lower in the sky?
(c) Would someone in Cairo, Egypt, which is at 30.0° North latitude, see the same sky
as you tonight? Why?
(d) Would someone in Sydney, Australia, which is at 33.9° South latitude, see the same sky as you
tonight? Why?
(a) The North star is at your zenith (directly above you):
(b) All the stars rise and set:
(c) Some stars circle the North star in sky:
(d) The North star is on your horizon:
(e) In the course of a year, all stars are visible:
(f) The North star appears at an altitude of 30° above your horizon:
