Friday, June 20, 2008

Last Day of Spring & First Day of Summer

In the United States and the rest of the northern hemisphere, the first day of the summer season is the day of the year when the Sun is farthest north (on June 20th or 21st). This day is known as the Summer Solstice.

The declination of the Sun on the Summer Solstice is known as the tropic of cancer (23° 27'). In the southern hemisphere, winter and summer solstices are exchanged so that the Summer Solstice is the day on which the Sun is farthest south.

A common misconception is that the earth is further from the sun in winter than in summer. Actually, the Earth is closest to the sun in December which is winter in the Northern hemisphere.

As the Earth travels around the Sun in its orbit, the north-south position of the Sun changes over the course of the year due to the changing orientation of the Earth's tilted rotation axes. The dates of maximum tilt of the Earth's equator correspond to the Summer Solstice and Winter Solstice, and the dates of zero tilt to the Vernal Equinox and Autumnal Equinox.

The reason for these changes has to do with the Earth's yearly trip around the sun. For part of the year the Earth's North Pole points away from the sun and part of the time toward it. This is what causes our seasons. When the North Pole points toward the sun, the sun's rays hit the northern half of the world more directly. That means it is warmer and we have summer.

The day of the summer solstice is the longest day of the year. The length of time elapsed between sunrise and sunset on this day is a maximum for the year. In the United States, there are about 14½ hours of daylight on this day.

UTC date and time of solstices and equinoxes
year Equinox
day time day time day time day time
2002 20 19:16 21 13:24 23 04:55 22 01:14
2003 21 01:00 21 19:10 23 10:47 22 07:04
2004 20 06:49 21 00:57 22 16:30 21 12:42
2005 20 12:33 21 06:46 22 22:23 21 18:35
2006 20 18:26 21 12:26 23 04:03 22 00:22
2007 21 00:07 21 18:06 23 09:51 22 06:08
2008 20 05:48 20 23:59 22 15:44 21 12:04
2009 20 11:44 21 05:45 22 21:18 21 17:47
2010 20 17:32 21 11:28 23 03:09 21 23:38
2011 20 23:21 21 17:16 23 09:04 22 05:30
2012 20 05:14 20 23:09 22 14:49 21 11:11
2013 20 11:02 21 05:04 22 20:44 21 17:11
2014 20 16:57 21 10:51 23 02:29 21 23:03

In astronomy, equinox can have two meanings:

  • Either of the two events when the Sun is positioned directly over the Earth's equator and, by extension, the apparent position of the Sun at that moment, or the time that it happens.
  • The time at which the vernal point, celestial equator, and other such elements are taken to be used in the definition of a celestial coordinate system.

An equinox in astronomy is that moment in time (not a whole day) when the centre of the Sun can be observed to be directly above theEarth's equator, occurring around March 20 and September 23 each year.

More technically, at an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points—the vernal point and the autumnal point. By extension, the term equinox may be used to denote an equinoctial point.

There is either an equinox (autumn and spring) or a solstice (summer and winter) on approximately the 21st day of the last month of every quarter of the calendar year. On a day which has an equinox, the centre of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth and night and day will be of nearly the same length. The word equinox derives from the Latin words aequus (equal) and nox (night). In reality, the day is longer than the night at an equinox. Commonly the day is defined as the period that sunlight reaches the ground in the absence of local obstacles. From Earth, the Sun appears as a disc and not a single point of light; so, when the centre of the Sun is below the horizon, the upper edge is visible. Furthermore, the atmosphere refracts light; so, even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. In sunrise/sunset tables, the assumed semidiameter (apparent radius) of the sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc. Their combination means that when the upper limb of Sun is on the visible horizon its centre is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. These effects together make the day about 14 minutes longer than the night at the equator, and longer still at sites toward the poles. The real equality of day and night only happens at places far enough from the equator to have at least a seasonal difference in daylength of 7 minutes, and occurs a few days towards the winter side of each equinox.

To summarize: Happy Summer!


No comments: