University of Michigan - Department of Astronomy

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Version: motions


Motions in the Sky

It is always sunrise somewhere... Eternal sunrise, eternal dawn and gloaming, on sea and continents and islands, each in its turn, as the round earth rolls.

--John Muir

Overview

Introduction

Understanding the seasons, timekeeping, reading starcharts, or knowing which stars will be visible from any location at a particular time all depends on understanding the motions of the sky. To make this a little easier we impose some guides on the sky.

The celestial equator and celestial poles are the projection of Earth's equator and poles onto the sky. The stars will appear to rotate around the celestial poles just as Earth's poles are the axis of Earth's rotation. These guides are in the same location for observers anywhere on Earth. For example, the star Polaris is very close to the north celestial pole, so no matter where you are on Earth the north celestial pole is close to Polaris.

There are also guides that depend on the observer's location. Zenith is the point directly overhead. The line that runs north and south and passes through zenith is the meridian. An object is said to transit when it crosses the meridian, and it will be at its highest point for the day at transit.


Part 1: Daily motion in Ann Arbor

The instructor will set the planetarium to display the sky from Ann Arbor tonight, just before sunset.

  1. Where is the Sun setting (circle one)

Due W, WNW, NW, NNW, WSW, SW, SSW

  1. What constellation is on/near the horizon in the south?  Enter the name in table 1.
  2. Identify a constellation  near the horizon in the SE, E, NE, and on the N horizon.  Enter their names in table 1.
Start Loc Constellation Name Loc +2h Loc +4h
S horizon      
SE horizon      
E horizon      
NE horizon      
N horizon      
  1. Your instructor will run the planetarium forward 2 hours.  Watch the constellations you recorded in table 1 to see where they go.  Record their new positions in the loc +2h column of table 1, including whether they are on the horizon, low, high, etc.  For example, if a constellation is just over half way between the E horizon and zenith, record its position as “E, high”. If the constellation is directly over the projector, record its position as “zenith”.  If it is no longer visible, record its position as “set”.
  2. Your instructor will run the planetarium forward another 2 hours.  Record the positions of the constellations again, as in the previous set, in the column labeled “loc +4h”
  3. Your instructor will let the planetarium move slowly forward.  Answer the questions below, checking your answers by observing the planetarium.
  4. Looking north, sketch the path of a couple stars.
  5. Looking south, sketch the path of a couple stars.
  6. Looking east, sketch the path of a couple stars.
  7. Do the stars that rise due east pass through zenith?



Part 2: Annular Motion in Ann Arbor

Your instructor will move forward one month and tell you the date.

  1. Date:

  2. How high is the Sun when it transits (read the number off the meridian)

  3. What constellation is it in when it transits?

  4. Where is the Sun setting?

  5. What constellation is it in when it sets?

Your instructor will move forward another month and tell you the date.

  1. Date:

  2. How high is the Sun when it transits (read the number off the meridian)

  3. What constellation is it in when it transits?

  4. Where is the Sun setting?

  5. What constellation is it in when it sets?

Answer the following questions based on your observations

  1. Does the Sun always pass through zenith from Ann Arbor?  If not, does it ever pass through zenith?  If yes, how many days and if no does it pass north or south of zenith?

  2. Is the Sun always on the celestial equator?

  3. How many constellations will the Sun move through in one day?

  4. How many constellations will the sun move through in one month?

  5. Where do you look to see the celestial equator?

Part 3: Daily Motions of the Sun and Stars from other Locations

Your instructor will move the planetarium to show the sky at the equator.  Try to keep an eye on a couple constellations as the projector moves.  Once at the equator, your instructor will set the projector running to show the stars rising and setting.  Make sure you watch one full day’s rotation, then answer the following questions.

  1. Where do you look to see the celestial equator?

  2. Are there stars visible from Ann Arbor that are still visible here?  If yes, are there any you can’t see anymore?  Are there any you can see now that you can’t see in Ann Arbor?


  3. Looking north, sketch the path of a couple stars.
  4. Looking south, sketch the path of a couple stars.
  5. Looking east, sketch the path of a couple stars.
  6. Do the stars that rise due east pass through zenith?

  7. Are there any circumpolar stars?  If yes, where are they?

Your instructor will move the planetarium to show the sky from 42º South.  Try to keep an eye on a couple constellations as the projector moves.  Once at 42º S, your instructor will set the projector running to show the stars rising and setting.  Make sure you watch one full day’s rotation, then answer the following questions.

  1. Where do you look to see the celestial equator?

  2. Are there stars visible from Ann Arbor that are still visible here?  If yes, are there any you can’t see anymore?  Are there any you can see now that you can’t see in Ann Arbor?


  3. Looking north, sketch the path of a couple stars.
  4. Looking south, sketch the path of a couple stars.
  5. Looking east, sketch the path of a couple stars.
  6. Do the stars that rise due east pass through zenith?

  7. Are there any circumpolar stars?  If yes, where are they?


Part 4: Ecliptic, Equator, and motions of the moon and planets

Your instructor will now go back to Ann Arbor and turn on the Moon and planets, as well as the ecliptic and equator.

  1. What is ecliptic? 

  2. What is the equator?

  3. Which line do the zodiac constellations lie on?

  4. What line do the planets and moon lie close to?

Your instructor will run forward until moon rise

  1. What constellation is the moon in when it rises?

Point out a visible planet, preferably close to moon.

  1. Is this an inner or outer planet?

  2. What constellation is the planet in?

Your instructor will run forward until the moon transits.

  1. What constellation is moon in? 

  2. What constellation is the planet in?

Your instructor will run forward to moon set.

  1. What constellation is moon in?
  2. What constellation is the planet in?

  3. How many constellations does the moon move through in one night?

  4. Based on this, how many do you expect it to move through in 1 month?  Explain


 

 


updated: 9/7/07 by SAM

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