Spring, Astronomically Speaking…

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by Pat Browne

Question: What is the Sun’s Declination On March 20 (Spring) 2017?

You may be asking this question as we celebrate the arrival of astronomical Spring on Mar 20 of this year (2017). You may first ask the question what does the term Declination mean? Let’s answer  this question first. The term ‘Declination’ in astronomy specifies one of the two coordinates on the Celestial Sphere… We refer to Right Ascension, Declination (RA, DEC) coordinates in astronomy, the same way our GPS refers to Longitude, Latitude (LON, LAT) on the map.

The Celestial Sphere – coordinates for ‘spaceship’ earth:

Celestial Sphere extends outward from the terrestrial sphere (Earth) Grid of RA, Dec lines in blue/green Path of the Sun in red

The celestial sphere is an imaginary sphere of arbitrarily large radius concentric with Earth. All objects in the observer’s sky including the sun can be thought of as projected upon the inside surface of the celestial sphere, as if it were the underside of a dome or a hemispherical screen. The celestial sphere is a practical tool for allowing observers to plot positions of objects in the sky including the sun.

The Celestial Coordinates are like terrestrial coordinates and are given the name Equatorial Coordinates (extending from our terrestrial equator).

  • Longitude in the sky = Right Ascension or RA
  • Latitude in the sky = Declination or DEC
-diagram courtesy Stephen J. McIntyre
courtesy Sky and Telescope

The sun travels along the Celestial Sphere and traces the trajectory of the path of the Ecliptic. The Sun’s position when its trajectory intersects the celestial equator corresponds to the Declination = 0 position. You can see this point on the diagrams above and below. At this point of intersection, we have our astronomical ZERO for Declination. This answers our question.

The Right Ascension coordinate  of the Spring Equinox also happens to be  0H and this position is called the First Point of Aries: The “First Point of Aries” is the location of the vernal equinox, and is named for the constellation of Aries.(However due to the  precession of the earth’s axis,  it is now located in the constellation Pisces). It is one of the two points on the celestial sphere at which the celestial equator meets the ecliptic plane. For the Fall equinox, the Right Ascension is 12 H, but the Declination is again 0.

When the Sun’s path intersects the Celestial Equator we experience a day of equal amount of sunlight day and night. Hence we call it Spring Equinox.

During Spring and Autumnal Equinox, (dates actually vary) the earth’s axis is tilted neither towards or away from the sun.

For 2017, the Spring Equinox occurs at 6:29 a.m. EDT in the northern hemisphere. Looking at our Star in the sky at high noon  (~1pm local EDT time), we can see that it is pretty near 0 deg Declination (and Right Ascension):

-Astronomically Spring  RA = 0 Hrs, Dec = 0 Deg     (image courtesy Stellarium)

To learn more about the celestial system see the Night Sky course notes on Locating Stars