The Vernal Equinox

On March 20th, 2015, the northern hemisphere's spring season officially began. This time of year is called the vernal equinox, or spring equinox. We've discussed equinox before and we learned that equinox comes from the Latin for "equal night". It just means that there are the same number of hours when the Sun is above the horizon (day) as there are when the Sun is below (night).

We have discussed the autumnal equinox before, and we learned that this is when the Sun appears to travel from north to south along the ecliptic across the celestial equator. In this case, the vernal equinox occurs when the Sun travels across the celestial equator from south to north along the ecliptic. You should note that when the spring starts in the northern hemisphere, fall or autumn begins in the southern hemisphere.

Another fun thing to note is that on either equinox, the Sun rises due east and sets due west. At the equator, the Sun is directly overhead at noon. As the Sun travels farther north on the ecliptic and we get deeper into the northern spring, the Sun will be farther north in the sky at noon until it reaches its farthest point north at the summer solstice (which will be discussed as we approach that time of year).

Celestial Sphere

The celestial sphere is the imaginary sphere covering the sky surrounding the Earth with the Earth at the center of the sphere. It is a way to map the location of stars, galaxies, and extra-solar objects in the sky, as well as track the movements of the Sun, Moon, and planets among the background stars.

There are a few important locations on the sphere that should be defined to help get a sense of what to look for in the sky.

Zenith: the point on the celestial sphere directly above your location on Earth. This point is dependent on where you are on Earth. To someone in Pittsburgh, PA, USA, their zenith is not that same as to someone in Sydney, New South Wales, Australia.

Nadir: the point on the celestrial sphere directly below your location on Earth. Like the zenith, this is location dependent.

The North (South) Celestial Pole: the point on the celestial sphere directly above the North (South) Pole on Earth. The North (South) Celestial Pole is the zenith for the North (South) Pole and the The South (North) Celestial Pole is the nadir for the South (North) Pole.
 
The Celestial Equator: similar to the equator on Earth. In fact, it is the imaginary line on the celestial sphere is directly above the Earth's equator. It is used to help explain the equinoxes as well as the solstices.

Celestial Prime Meridian: the line on the celestial sphere equally 0h Right Ascension. It is the line directly corresponding to 0° longitude on Earth, i.e. the prime meridian running through Greenwich, England.

Right Ascension: equivalent to longitude on Earth, this is an angle measured in hours, minutes, and seconds from 0h to 24h. The angle increases from east to west, from the Celestial Prime Meridian

Declination: equivalent to latitude on Earth, also measured in degrees, arcminutes, and arcseconds from 0° to 90° from the Celestial Equator to the Celestial Poles. Travelling north from the Celestial Equator to the North Celestial Pole, the angles are positive, and travelling south from the Celestial Equator to the South Celestial Pole, the angles are negative.

Ecliptic: this is defined in another page, located here. It is the imaginary path the Sun traces over a course of the year on the Celestial Sphere. It is tilted at 23.5° with respect to the Celestial Equator and when it crosses the Celestial Equator, the equinoxes occur. At the Sun's northernmost point on the ecliptic, we have the summer solstice in the Earth's northern hemisphere and the winter solstice in the southern hemiphere. At the Sun's southermost point, the solstices are switched.