Western Washington University   College of Sciences and Technology   Physics/Astronomy Dept.   Bellingham, WA USA
 
ASTRO 101

Analemma
Asteroids
Aurora
Big Bang
Black Holes
Bode Titius
Brightest Stars
Comets
Constellations
Coordinates
Cosmology
Cruithne
Dark Matter
Eclipses
Galaxies
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HR Diagram
Hubble's Law
Intelligent Life
Kepler's Laws
Leap Year
Light Waves
Lunar Libration
Messier Objects
Meteors
Milky Way
Moon
Moon Phases
Planets
Precession
Rainbows
Redshift
Seasons
Stellar Evolution
Stardust
Sun & Fusion
Telescopes
Tides
Time of Day
Twilight
Zodiac
 
The Analemma

Ever notice on some globes there is a strange figure 8, with tick marks and dates, usually printed somewhere in the middle of one of the oceans? It's called the analemma figure and here is what it is all about. The analemma shows the latitude on Earth where the Sun's rays are vertical on a given day of the year, and the "equation of time" which tells whether local clocks are ahead or behind solar time.

Equation of Time

The solar day is the length of time between one local noon, when the Sun crosses the meridian, to the next. But the length of the solar day is not always 24 hrs - its average is 24. The average is called Mean Time.

The Earth's axis is tilted which works to move the Sun's relative position north and south, hence the seasons. But the orbit around the Sun is elliptical instead of a circle so our planet moves faster at perihelion than at aphelion, which works to move the position of the Sun slightly east and west of the average position. The north-south motion is the height of the figure 8 and the east-west motion is the width of the figure 8. The effects combine to create an everchanging offset between Mean Time and the appearance of local noon by as much as +/- 16 minutes. This difference is called "the equation of time."

Because of the changing equation of time, if you...

  • take lots of pictures of the Sun
  • each from the same location on Earth
  • always at the same time of day
  • at intervals for a whole year
  • all on the same piece of film
This is the sort of image you will get.

Credit & Copyright: Vasilij Rumyantsev