When Day = Night

    The spring season for the Northern Hemisphere starts around March 20th.
    What does it mean when we say that a season starts? The answer to this question, in part, is based on a system of grid lines that, while similar to latitude and longitude, are not on the surface of the Earth. These lines are the celestial, or sky, grid and are literally an extension of the Earth-surface grid system into the sky. When we can extend the Earth’s equator skyward, it becomes the sky equator, dividing the sky around us into a northern and southern half. Further divisions of the sky north and south of the sky equator are known as lines of declination, whereas on Earth these are parallels of latitude. The celestial sphere is also divided into 24 sections similar to meridians of longitude. However, these lines are known as hour circles of right ascension, or hour circles. Hour circles are numbered from 0 to 23. The intersection of these hour circles and lines of declination determines a celestial object’s coordinates on the celestial sphere.
mar-equinox   For example, (using the northern hemisphere) the Sun passes through the 0-hour circle at the start of spring, and it passes through the 12-hour circle at the start of fall. Additionally the Sun’s position relative to the sky equator at this time, its declination, is 0-degrees – the Sun is ‘on’ the equator. This information, the object’s hour circle and declination, are enough to locate its position on the celestial sphere. However, an additional line can be added to the celestial sphere to show the daily changes in the Sun’s position in the sky throughout the year. This line, known as the ecliptic, represents the path the Sun seems to follow throughout a year. The Sun seems to move eastward along the ecliptic, with respect to the stars in the background, because of the Earth’s orbital motion about the Sun.
   This graphic shows a portion of the celestial sphere and grid system of hour circles and lines of declination. The Sun is located at the point where the ecliptic crosses the sky equator. At this intersection, the Sun has coordinates of 12 hours, 0 degrees. This, then, is when northern hemisphere fall starts. While these figures give its coordinate position, the Sun can also be described in terms of the constellation it is in (actually in front of), as viewed from Earth. Thus, at the start of northern hemisphere spring (March) the Sun is in Pisces the fishes, while in the fall (September), we would say the Sun is within the boundaries of Virgo.
   “Equinox” is derived from the Latin, meaning equal night, and when it occurs, the Sun rises due east and sets due west everywhere on Earth except near the poles, giving us 12 hours of daylight and 12 hours of nighttime. During the summer, the Sun rises in the northeast and sets in the northwest, and during the winter, the Sun rises in the southeast and sets in the southwest in the Northern Hemisphere.