Feature: Calculating Vernal Equinox

March 20, called vernal equinox in the northern hemisphere, marks the beginning of the astronomical year.

It's the point where the Earth's equator (and by extension the celestial equator) is crossed by the ecliptic, the path which the Sun follows in front of the background stars, in other words, it is the moment at which the center of the visible Sun is directly above the Equator or, when the longitude of the Sun is 0° and 180°. Traditionally, this is called "the first point in Aries," which actually lies today in the constellation Pisces.

What happens on March 20 is that the Sun moves from south of the celestial equator to north of the equator. (In reality, it is the Earth that is moving, and not the Sun, but from our perspective it seems the other way around.) The days, which have been getting longer since winter solstice on Dec. 21, for a moment equal the nights in duration all over the planet, which is the reason it is called "equinox" -- Latin for "equal night."

On the day of an equinox, the Sun rises everywhere on Earth (except at the poles) at 6 AM and sets at 6 PM (LST; local solar time).

Knowing our latitude, we can derive a few data points, and either calculate or visualize them on the horary quadrant.

  • Latitude = 52.5°
  • Co-latitude = 90° - latitude = solar maximum on equinox = 37.5°
  • 37.5° - 23.5° (declination) = 14° = solar maximum on winter solstice
  • 37.5° + 23.5° = 61° = solar maximum on summer solstice

We can also derive that sunrise is at 90° (due east), sunset at 270° (due west), and that at noon, the Sun is at exactly 180° (and vice versa).

(The title picture shows the reading of the unequal hours on a quadrant by setting the co-latitude (90° - latitude) and moving the cursor to the Sun maximum altitude at noon for the day of the equinox.)