*Motions are with respect to the stars, except for the diurnal motion, which is with respect to the local horizon.

• Intrinsic motions of the objects themselves with respect to one another

• The motion of the observer, or the platform on which he/she is standing---in this case, the Earth

B. EFFECTS OF EARTH SHAPE AND SPIN

• Earth is a sphere.

• Night = shadow side (away from Sun). Half of Earth is always in shadow.

• Spins on axis (with respect to the stars) once in 23 hours, 56 minutes (one "sidereal day"); ==> diurnal rotation of sky

• Spin counterclockwise (eastward) seen from above N pole; ==> apparent rotation of sky is westward

• Local "horizon plane" is plane "tangent" to Earth at your location; you can see (in principle) objects above plane but not below; the horizon plane sweeps across sky as Earth spins, and determines rising, setting of objects

C. EFFECTS OF EARTH'S MOTION IN ORBIT

• Earth is a planet in orbit around Sun

• Orbit is nearly circular and lies in a plane called the "ecliptic" plane (Fig.1-11)

• Orbits Sun in 365.25 days (one year) at ~66,000 mph; counterclockwise from above N. pole

• Stars visible at night are those "opposite" the Sun

• Apparent motion of Sun against stars (1 degree per day eastward) is the reflex of Earth's motion in orbit
  The apparent eastward motion means that the Earth must spin a little more than once on its axis to bring the sun back to the point of local "noon" (i.e. halfway between rise and set). The extra amount is 4 minutes, on average. This accounts for the difference between the sidereal rotation period of the Earth (23 hours, 56 min) and the average difference between sucessive noons (24 hours, the "solar day").

• The Sun's location on the sky as seen from Earth is confined to the ecliptic path = projection of ecliptic plane on celestial sphere; "zodiac" = constellations through which ecliptic passes.
  Given the modern boundaries of the constellations, there are 13 of these, as in Table 1-1 of the text. However, in classical astronomy (and current-day astrology) there are only 12---one for each month.

D. EFFECT OF TILT OF EARTH'S AXIS

• The polar rotation axis of Earth is fixed in direction with respect to the stars, not with respect to the Sun

• The axis is not perpendicular to the ecliptic (23.5 degree tilt); equivalently, the ecliptic is inclined to the celestial equator. (Figs. 1-11, 1-15).

• Sun therefore moves north and south of the celestial equator during the year. Total amplitude of swing = 2 x 23.5 = 47 degrees.

• "Equinoxes": Sun on celestial equator; "solstices": Sun at greatest distance from equator.
  • At equinox (Mar 21, Sep 21), night and day are each 12 hours long at all latitudes

  • At solstice (June 21, Dec 21), one hemisphere experiences longest day, other shortest

• Sun N/S distance determines hours of daylight (i.e. location and shape of the shadow) and the angle at which sunlight strikes Earth's surface. It thus determines "insolation", or the amount of sunlight incident locally on the Earth during 24 hours, therefore atmospheric heating and the seasons
  The change in the shadow distribution on the Earth's surface caused by the tilt is quite dramatic. Here are two images of the way the shadow is distributed at about 2 PM Eastern time on August 1 (left) and December 1 (right). The shadow moves westward on the surface. You can immediately tell from the image which latitudes are receiving more sunlight in a 24 hour period. Click on the thumbnails for an expanded view.

  
  
  

• Seasons are therefore caused by tilt, not distance to the Sun. (Fig. 1-12, 1-14)

Interactive Earth & Moon viewer (includes software & add'l links)
JPL Tutorial on Earth motions and astronomical reference systems

Last modified 14 September 1998 at 19:00 EST by rwo

Back to the list of study guides

Back to the ASTR 121, Sec. 3 Home Page

---

Copyright © 1998 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia. Reproduction, distribution, and commercial uses are prohibited.