Celestial Sphere & Solar Motion Lab (Norton s Star Atlas pages 1-4)

Transcription

1 Name: Date: Celestial Sphere & Solar Motion Lab (Norton s Star Atlas pages 1-4) Italicized topics below will be covered only at the instructor s discretion. 1.0 Purpose: To understand a) the celestial sphere, including its equatorial coordinate system, the ecliptic, the zodiac constellations, and the equinoxes and solstices b) the path taken by the Sun during different times of the year and at different latitudes 2.0a Equipment: ruler, protractor, celestial sphere model, solar motion demonstrator (if not already assembled, will require instructions, pre-printed cardstock, brad [the Sun], glue, sharp scissors or exacto-knife with cutting board) [Instructor note: At least one solar demonstrator is needed for every pair of students. A large wooden solar demonstrator is in the back of SM 101.] 2.0b Theory and Investigation: A. Examine the celestial sphere and locate the Big Dipper. Try to visualize it by imagining that you are at the center of the globe looking out. This is the way it would appear in the night sky. B. Notice that the sphere is pivoted at the North Celestial Pole (NCP) and at the South Celestial Pole (SCP). These are the points in the sky resulting from the intersection of the Earth s Polar Axis and the Celestial Sphere. C. Notice that halfway between the celestial poles lies the Celestial Equator, the intersection of the plane of Earth s equator and the Celestial Sphere. D. Notice that the Moon and planets are not shown on the globe. They do not have fixed positions on the Celestial Sphere. E. Locate the Sun, the gold/yellow ball, and rotate it noting the apparent path it takes as viewed from Earth. Notice that this path, called the Ecliptic, is inclined 23½ to the Celestial Equator which causes our seasons. The Ecliptic might also be thought of as the projection of the Earth s orbit onto the Celestial Sphere.

2 F. Locate the months and days of the year that are marked off along the Ecliptic so that the position of the Sun can be known on any day of the year. Find the position of the Sun today. G. Move the Sun Eastward (or counter-clockwise as viewed from North) along the Ecliptic, until it eventually crosses the Celestial Equator going from the southern half to the northern half of the sky. [*If you have studied seasons, think about whether the sunlight is becoming more or less direct on the Northern Hemisphere during the time that the Sun passes from the southern to northern half of the sky. Without reading the dates, try to determine if this would be the Spring or Fall Equinox?] H. Read the date of this point of intersection off the celestial sphere to see if it agrees with the Vernal (Spring) Equinox, which is where the Sun arrives around March 21. I. Now look at the path taken by the Sun around the Autumnal (Fall) Equinox, noting the date of that event and how it compares to where the Sun arrives around September 21. See * above. J. Unlike the Celestial Sphere, the solar motion demonstrator can be adjusted to simulate the apparent daily path taken by the Sun during any month for various Northern latitudes. Set the device to our latitude (33.5 degrees), noting your horizon (the disk). Move the Sun (the metal brad) to various months (e.g. the equinoxes and the solstices) then sweep the sun across the sky from Eastward to Westward, noticing (1) the path taken, including exactly where it rises and sets (2) if it tends toward the North, the South or goes directly overhead, when it is highest in the sky, and (3) if it takes a long, medium or short path through the sky. Other latitudes will be studied in the exercises below. 3.0 Procedure: Answer all of the questions below in the space provided, consulting with partners as needed. Following completion, there will be an in class discussion of the answers. We will then bring your celestial spheres to the roof and await further instruction. (1) Move the Sun until it appears to be at a point on the Ecliptic that corresponds to your birthday. a) Write your birthday and the Constellation you find this point to be in. b) Is this the sign you usually associate with your birthday? c) If not, what is? d) What Zodiac constellation(s) would you see approximately highest in the night sky on your birthday? (Hint: What Zodiac constellation(s) are on the opposite side of the Earth as the Sun, i.e. where the Sun doesn t shine?) (2) Find these on the Celestial Sphere and then record the corresponding dates: a) when the Sun is at its highest point over the northern celestial sphere, b) when the Sun is at its lowest point over the southern celestial sphere c) when the Sun is on the Celestial Equator (two dates here).

3 (3) Find the Declinations (recall that this is similar to latitude, but projected onto the celestial sphere) of the following regions: a) the Celestial Equator, b) the North Celestial Pole, and c) the South Celestial Pole. (4) Use the solar motion demonstrator to examine the daily path of the Sun through the sky at our latitude during the equinoxes and the solstices. Specifically, during each of these four times, a) where does the Sun rise and set (there will be 2 answers for each of the 4 times studied, e.g., during the solstice, the Sun rises in the southeast and sets in the southwest, etc.)? b) does the Sun tend toward the north, the south or go directly overhead when it is highest in the sky (4 answers)? c) how does the length of a day (and a night) compare for each of these times, consider the length of the Sun s path through the sky (4 answers)? [Hint: Consider the meaning of the word equinox and comment on how that meaning relates to the answer to this question.] Using the solar motion demonstrator to examine other latitudes. (5) (a) At what latitude(s) and during what months, would the Sun rise directly in the east, set directly in the west and pass directly overhead? (Recall that directly overhead is also called your.) (b) At what other latitude(s) and during what months, would the Sun pass directly overhead (but not necessarily rising due east or setting due west)? Try to find a range of latitudes that work.

4 (c) In FIGURE 1,below, find Earth s axis of rotation, then draw a long line perpendicular (at 90 degrees - use the protractor or a right angle template) to this and passing all the way through the Earth s center; this line should be on top of and longer than the existing line labeled Celestial Equator. Find the Ecliptic in FIGURE 1, recall that it lays along the orbit of the Earth about the Sun. Carefully draw a long line, perpendicular to the Ecliptic and passing through the Earth s center; label this line perpendicular to Ecliptic. Now label the angle between Earth s axis and perpendicular to the Ecliptic as 23.5 degrees, the tilt angle. Try to determine the angle between the Ecliptic and the Celestial Equator and label it. If you aren t sure, then use a protractor to measure this angle and proceed to label it on the diagram and write it below. Is it close to any other angle that you know of? Can this angle explain the range of latitudes you found in part (b) above (for which the Sun would pass directly overhead)? If you are struggling with answering this question and/or part (b) above, try studying latitudes between degrees and 23.5 degrees, but realize that the solar demonstrator deals only with positive latitudes. FIGURE 1. (6) Examine the Sun s apparent motion at our latitude and the equator. How does the length of the Sun s path vary and will its path produce direct or indirect light or something in between. (a) Can you see why the winter climate near the equator is milder than ours? (b) Now take a look at the year round climate at the equator, as compared to our climate (looking at path and angle of sunlight). Explain why the climates vary. (7)During what month(s) would it be dark when living at the North Pole?

5 (8) Find the Right Ascension line marked 0 Hours. Note a) where it crosses the Celestial Equator. b) what do we call that point? Place the Sun at this point. c) record the approximate Date and the Right Ascension and Declination of the Sun. (9) Move the Sun along the Ecliptic. Find the approximate Dates, Right Ascensions and Declinations of the Sun when it is at the a) Vernal Equinox, b) Summer Solstice, c) Autumnal Equinox and d) Winter Solstice. (12 answers here) (10) Why do the Constellations change from season to season. (Hint: Earth s tilt is not the answer.) (11) Imagine two observers, one at the North Pole and the other at the South Pole of the Earth. Which stars can they both see? (Assume a very slight overlap due to atmospherics.) (12) Where would an observer on Earth need to be located in order to see approximately all of the stars in the night sky over a year? (See The Practical Astronomer, page 33, for an observer s view and some star trails for various latitudes.) (13) Draw a diagram showing why the Sun is higher in the sky at the Summer Solstice than at the Winter Solstice.