Equation Of Time

amateur  astronomy series #3

The Analemma of Time

 The equation of time (The word "equation" is here used in a somewhat archaic sense, meaning "correction". Prior to the mid-17th Century, when pendulum-controlled mechanical clocks were invented, sundials were the only reliable timepieces, and were generally considered to tell the right time.) is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer on Earth.

 Apparent (or true) solar time can be obtained for example by measurement of the current position (hour angle) of the Sun, or indicated (with limited accuracy) by a sundial. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time average to zero (with zero net gain or loss over the year).

The equation of time is also the east or west component of the analemma, a curve representing the angular offset of the Sun from its mean position on the celestial sphere as viewed from Earth. The equation of time values for each day of the year, compiled by astronomical observatories, were widely listed in almanacs and ephemerides.

as in Northern hemisphere

 The equation of time commonly is represented in diagramatic form as an anlemma of time showing both solar declination and the difference between true and mean solar time over the course of a year. The diagram portrays the approximate equivalence of true solar with mean solar time in mid-April, mid-June, the end of August and late December. Solar time leads by around 2 minutes at the December Solstice, 4 minutes in mid May , 7.5 at the September Equinox and 16 minutes in early November. It lags by around a minute at the June Solstice, 7.5 minutes at the March Equinox, 6 at the end of July and 14 minutes in mid February.

Equinox (equal night)

Two instances each year at which the Sun appears to cross the celestial equator. The spring or Vernal Equinox occurs around March 21, when the sun is overhead at the Earth's equator, crossing from south to north. The Autumnal Equinox occurs when it crosses from north to south, around September 23. At the time of equinoxes, day and night are of equal length all over the world, the sun rising due east and setting due west on those days.

The Earth in its orbit around the Sun causes the Sun to appear on the celestial sphere moving over the ecliptic (red), which is tilted on the Equator (white).

Solstice: word solstice is derived from the Latin sol (sun) and sistere (to stand still)

The time when Sun is at it's greatest declination, 23.5 N or 23.5 S, marking the northern ( northern solstice) and southern limits of it's annual path along the ecliptic. In the Northern Hemisphere the summer solstice occurs around June 21, when the sun reaches it's highest altitude in the sky and is overhead at the  Tropic of Cancer  ( Cancer). This marks the longest day of the year, the period of maximum daylight. The winter solstice occurs around December 22, when the sun reaches it's lowest altitude, being overhead at the  Tropic of Capricorn (Capricorn). This is the point of the shortest day, when daylight hours are at a minimum. The solstices are reversed in the Southern Hemisphere 

 The Latin names Hibernal solstice (winter) and Aestival solstice (summer) are sometimes used.

Illumination of Earth by Sun at the northern solstice.

Illumination of Earth by Sun at the southern solstice.

*Note: all pictures thankfully shared from various sources..