Risings and Settings - Location

Variability of Rising/Setting Location

In contrast to fixed stars, a given planet (including the Sun and Moon) will not always rise and set at the same location on the horizon. Sometimes a planet will rise at a more southerly location; sometimes at a more northerly location (though keeping within a certain range). 

How does the Sun's apparent zodiacal motion provide ways to make a calendar? 

The Sun traces its rising and setting positions north and south along the horizon in an annual pattern that is exactly repeatable and that recurs seasonally. Thus it provides a reliable and convenient calendar, like a pendulum oscillating around the points due east or due west. This solar calendar was as significant for ancient cultures as the cycles of day to night and the lunar phases. For example, an ancient Chinese explanation of this pattern attributed the motion to the Earth, as if the Sun were rising each day at the same place but the Earth itself slowly sliding up and down along a north-south line. At Stonehenge (Britain), Woodhenge (Cahokia, Illinois), or other stone circles (e.g., Medicine Wheel, Wyoming), seasonal time intervals were charted by arranging sunrise markers sighted toward the eastern horizon, running north or south of due east. The average American today, though ignorant of the term "zodiacal motion" knows that the Sun appears to go through a cycle that is the basis of the "year" in our calendar (postpone considering for now whether the earth goes around the Sun or the earth). 

What are the principal positions (events) in the sun's exactly repeatable zodiacal motion? How do these main events show up on a calendar? 

Sunrises and sunsets reach their extreme northerly and southerly positions on the solstices; and occur due east and due west on the equinoxes.

Difficulties of ascertaining rising phenomena

The position of the Sun on the horizon at sunset or sunrise is difficult to determine accurately and consistently. As we continue to investigate the zodiacal motion of the Sun (its annual cycle which is the basis of our solar calendar), we need to focus our attention on the problem how to determine the points on horizon at which the Sun rises on successive mornings. The exact rising position of the Sun might be crucial for an ancient culture's determination of the 4 seasons of the year (and thus religious and agricultural events as well). At first glance, one may wonder what is so complicated about such rising (or setting) phenomena so as to warrant its inclusion here. To see some of the problems, consult the appendix in Crowe, and the diagrams he offers on p. 204 and p. 214. 

Other factors complicate this phenomenon as well, including local lighting and terrain and the star's magnitude--the latter contributes to starlight refraction errors, caused by the Earth's atmosphere, that affect the star's apparent rising or setting position. In Mesopotamia (where rising and setting events were of utmost significance) sand-storms could also be expected to obscure the horizon and to frustrate accurate sightings (only Babylonian eclipse records--which were not horizon phenomena--proved sufficiently reliable to later astronomers such as Ptolemy). 

 

Study Questions

  • Equinoxes are defined ... 
    • ___ by horizon phenomena (the Sun rising due east and setting due west) 
    • ___ as dates calculated to be midpoints in time between the solstices 
    • ___ both of the above 
  • How might one define the moment of sunrise? List three different possibilities. 

 

  • "The tropical year is longer than the sidereal year." True or False? 
  • The phenomenon of unequal seasonal lengths has interesting implications, assuming a spherical Earth, for the seasons of the opposite hemisphere. Imagine two observers, Sally in the Southern hemisphere and Nick in the Northern hemisphere. For each of the following, circle the season that is longer: 
    • Nick's winter; Nick's summer 
    • Sally's winter; Sally's summer 
    • Nick's winter; Sally's winter 
  • Obtain a globe and find the Earth's equator. Tilt the globe 23.5 degrees if its stand does not already tilt it at that angle. Use a flashlight or other object to represent the Sun. Explore what happens in the two cases below: 
    • 1) Leave the Earth still, and move the Sun around the Earth, sliding the Sun in a single plane inclined to the equator of the Earth. Does the Sun appear to cross the equator? 2) Leave the Sun still, and move the Earth around the Sun, sliding the Earth in a single plane inclined to the equator of the Earth. Does the Sun appear to cross the equator? Does the Earth wobble or see-saw in space? 3) Is there any observation an Earth-bound observer coud make that would be different in (1) as compared with (2)?