Reading: Astronomy, Chapter 2: Knowing the Heavens, sections 5-8
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.
The earth varies in distance from 91 million miles to 93 million miles over the course of its revolution around its slightly elliptical orbit, but this difference, less than 2% of the total distance from the sun, is not enough to cause the change in seasons, and in fact, the earth is closest to the sun aroun January 2, in the middle of the northern hemisphere's winter.
1 day = 24 hours = 360° rotation of celestial sphere.
1 hour rotation = 360/24 = 15° rotation celestial sphere.
1° rotation = (24 hrs * 60 min) / 360 = 4 minutes of time.
1 sidereal day = 23h 56m 4.091s
Sun's position March 21: 0°.
Sun's position June 20: 23.5° (north).
Difference in days: 91
Average change per day: 0.258°
Read the following weblecture before chat: The Earth in Space
Planetarium program exercise
The following picture set is based on observing projects #63-64-65 on p. 43 and was made using Starry Night. 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).
Sunrise, looking east, 22 Sept 2007
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.
The sun at noon....notice the faint celestial coordinate system markings.
The sun at noon -- but with the sky "dark" so that you can seed the background stars.
Notice the position of the sun on the ecliptic (green line), and at 0 ° declination, equidistant between -10° and +10deg; according to the grid to the right. The vertical lines are lines of RA, and marked along the top. 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.
The sun at noon 22 Sept 2007, with background constellations.
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.
Sunset 22 September 2007
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 Jupiter, just above the constellation Scorpio. 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.
The northern sky, 22 September 2007 as seen from Seattle (latitude 47°).
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
Planetarium picture made with Carina Voyager III for 16 September 2009, 9pm (DST).
Use the information at the US Naval Observatory site 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.
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.
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