Vernal Equinox 2016

Early Spring iris blooms in my garden (16 March 2016)

Yesterday at 12:30 AM (EST), Spring officially began for the Northern Hemisphere.  The date of the switch from Winter to Spring is known as the Vernal (Spring) Equinox.  On this date, the sun crosses over the celestial equator and begins to strike more directly on the Northern Hemisphere than the Southern Hemisphere.

If you were to rise each morning and observe the location of the sunrise from the exact same spot, you would find that the sunrise takes place in a slightly different location from one day to the next.  On the first day of Winter the sun would appear to rise far south of due east.

As winter progresses, the sun's position moves northward along the horizon a little bit each morning.  It will continue to move northward until the first day of Summer.  At that point each sunrise will move south along the horizon until the first day of Winter arrives again.  On the first day of Spring, the sun rises midway between these northern (Summer) and southern (Winter) maximums - the sun also rises at the midpoint on the first day of Fall (Autumnal Equinox).  Sunsets follow a similar pattern in the western sky. 

This shifting of the sunrise (and sunset) is due to the fact that the earth's axis is not directly perpendicular to the plane of its orbit.  If the earth's axis were perpendicular to the plane of its orbit, the sun would rise and set in the same location each day.  Instead the earth's axis is tilted approximately 23.5 degrees from the vertical - this is why globes are tilted.  For part of the year the Northern Hemisphere is tilted toward the sun (Spring and Summer) and for part of the year the Northern Hemisphere is tilted away from the sun (Fall and Winter).  Conversely, while the Northern Hemisphere is experiencing Summer the Southern Hemisphere is experiencing Winter; while the Northern Hemisphere is experiencing Winter the Southern Hemisphere is experiencing Summer.

Not only are the locations of the sunrise and sunset shifting, but the amount of daylight and darkness is also shifting.  On days when the position of the sun moves further north the amount of daylight grows slightly longer.  On days when the sunrise shifts southward the amount of daylight decreases slightly.

There are only two dates on which neither the Northern nor Southern Hemisphere is tilted more toward the sun.  On the equinoxes, both the poles line up with the direction of the earth's orbit and both hemispheres receive approximately equal amounts of daylight and darkness.

We are expecting snow later this week, but it officially Spring.  Get out there and enjoy it!