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Chapter 2: 5-8 Knowing the Heavens

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Scholars Online Astronomy - Chapter 2: 5-8 Knowing the Heavens


Reading Preparation

Reading: Astronomy, Chapter 2: Knowing the Heavens, sections 5-8

Study Notes: notes on your assigned reading from the text

Now we look at some of the consequences of the earth's position and orientation with respect to the sun and moon. Length of day changes as the earth moves around the sun because of the "tilt" of the earth's axis to the plane of its orbit. This changes the amount of sunlight reaching a given square foot of the earth's surface, creating differences in the amount of energy available to heat the atmosphere and oceans, or provide light for photosynthesis. The tilt and annual motion of the earth around the sun give earth its climates, seasons, and calendar.

Key Formulae to Know

Web Lecture

Read the following weblecture before chat: The Earth in Space

Study Activity

Planetarium program exercise

The following picture set is based on observing projects #63-64-65 on p. 43 and was made using Stellarium on my Mac. You should try to complete the observing projects on your own, but if you are unable to create similar views using your planetarium program, you can use the pictures below to make some observations.

Note: Because of the Earth's annual motion, this discussion and its pictions showing the position of the sun against the background sky is accurate within one day of 22 September for any year).


Look carefully at this picture of the sunrise. You should be able to see the celestial equator grid faintly in the background, and the ecliptic. Since the sun is at the autumnal equinox, the sun is near the position where the two lines (ecliptic and celestial equator) cross.


Notice the position of the sun on the ecliptic (orange line), and at 0 ° declination. The blue line is the celestial equator, and the sun is at 12hr RA, between 10hr and 14hr. Since it is noon, the 12hr RA line corresponds to the local celestial meridian -- and to local sidereal time.


This shows the same picture, but with the constellations drawn in. We say that "the sun is in Virgo", because it shows up against the background constellation Virgo, moving out of Leo and toward Libra. None of these constellations are visible for observing, since the sun's light is too bright.


This shows the sun at sunset, with the sky dark so that we can determine the other nearby objects. Mercury lies to the east of the sun in the sky at this time, and so sets later than the sun. Since it is below the ecliptic, it will be very near the horizon at sunset, and very difficult to see. The best time to find Mercury is at maximum elongation east (after sunset) or west (before sunrise), when it is north of the ecliptic.

Further to the east, you can see Venus, in the constellation Libra. It is bright enough to be visible in the SW sky at sunset and for about an hour after, before it, too, follows the sun below the horizon.


This shows the sky looking north. The lines of RA converge on the celestial north pole, 47° above the north point. Stars that are within 47 ° of the pole at 90° (in other words, with a declination of 90-47 = 43° or more) will be circumpolar, and always be above the horizon (although not visible during daylight hours).

Notes on Working with a Planetarium Program

  1. Study the map above. You are looking S on September 22, 2021, around 9 pm PDT, from Seattle, WA (47°N latitude, 122° W longitude).
  2. Find the green line: this is the horizon. It is marked with the directional points; S is to the left and NW to the right of this segment of the sky. Stars below the green line have already set.
  3. Find the orange line (passing through the Sun, near Spica, and through Antares). This is the ecliptic. The sun will move eastward along this path over the next months. What star does it pass near id mid-November? in August?
  4. Find the blue line (passing through the Sun, the West pont on the horizon, and going up near Altair). This is the celestial equator. Stars above and to the right have north declinations, stars down and to the left have south declinations. Note that the path of the sun for the next six months lies in the south declination.
  5. Find the planets -- this map shows Jupiter and Saturn due south, with Venus, Mars, and Mercury east of the sun. Are any of these planets planets at this time (above the horizon line? What about the Moon and Uranus? Which coordinate circle are they near, the ecliptic or the celestial equator?
  6. Try to create the same type of projection for today's date (whatever day you attempt this exercise) in your Stellarium program. You should use your own location. The sun should be near or below the western horizon, depending on your latitude. Where are the planets? are any visible? Where is the moon? Can you explain why the sun is in nearly the same part of the sky as it was on this date in 2009, but the Moon and planets may have changed position?

UNL Tools Exercises

  • Interactives: Work through the exercises unders the Seasons.
  • Class Action: Test your control of the concepts by answering the Questions for the Basic Motions and Ancient Astronomy module.
  • Labs:
    • Read through the materials for the Motions of the Sun, then use the simulator to see how the sun's position changes. Set the position for your latitude. How high is the sun above your south horizon at solar noon today?
    • Under The Rotating Sky: You might want to finish the sidereal time and hour angle demo to make sure that you know how sidereal time works.

Optional websites:

Use the links and information at the US Naval Observatory Astronomical Applications page to determine the time of sunrise, sunset, moonrise, moonset, and the phase of the moon for your location.

If you are still having problems envisioning coordinate systems, checkout the Thinkquest Project Coordinate System Unit. Thinkquest is a set of projects by students, for students.

Chat Preparation Activities

Chapter Quiz

Lab Work

Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.