First Days of Seasons 2011 2012: Spring / Fall Equinoxes; Summer / Winter Solstices

The astronomical dates and times for the first days of the seasons in 2011 and 2012 are given here in Coordinated Universal Time (UTC) for both Northern and Southern Hemispheres. To convert the times to your time zone, visit the World Clock HERE. Each season starts with either an equinox or a solstice. A simple explanation of what these are-along with the events that cause them-follows the time-and-date listings.

2011 First Days of the Seasons-Northern Hemisphere
Spring Equinox March 20 @ 23:21 UTC
Summer Solstice June 21 @ 17:16 UTC
Fall Equinox September 23 @ 09:04 UTC
Winter Solstice December 22 @ 05:30 UTC

2011 First Days of the Seasons-Southern Hemisphere
Fall Equinox March 20 @ 23:21 UTC
Winter Solstice June 21 @ 17:16 UTC
Spring Equinox September 23 @ 09:04 UTC
Summer Solstice December 22 @ 05:30 UTC

2012 First Days of the Seasons-Northern Hemisphere
Spring Equinox March 20 @ 5:14 UTC
Summer Solstice June 20 @ 23:09 UTC
Fall Equinox September 22 @ 14:49 UTC
Winter Solstice December 21 @ 11:12 UTC

2012 First Days of the Seasons- Southern Hemisphere
Fall Equinox March 20 @ 5:14 UTC
Winter Solstice June 20 @ 23:09 UTC
Spring Equinox September 22 @ 14:49 UTC
Summer Solstice December 21 @ 11:12 UTC

What Is an Equinox?
An equinox represents the time when the lengths of day and night nearly equal over all of Earth's latitudes-in reality, the day is slightly longer. It initiates a new season, either spring or fall.

What Causes an Equinox?
It's necessary to first understand that as the Earth orbits the Sun, completing one revolution in a year, we perceive the Sun as moving against the stars-an effect due to the Earth's rotation. As we track the Sun's apparent motion along an imaginary path called the ecliptic, we see the Sun the above the equator for a half a year and we see it below for the other half. Thus, the Sun appears to cross Earth's celestial equator (the projection of Earth's equator onto the sky) twice a year.

These apparent crossings occur at a time when the Earth's axis-which at present has a relative constant tilt of 23.5 degrees-leans neither toward nor away from the Sun. To re-state-the Earth's position in its orbit is such that neither the North Pole nor the South Pole leans either toward or away from the Sun. These conditions combined cause two annual equinoxes, six months apart.

When are the Equinoxes?
The equinoxes occur simultaneously in the Northern and Southern Hemispheres. However, they are opposing- spring v. fall. Vernal or Spring Equinox occurs in March for the Northern Hemisphere (all land above Earth's equator) and in September for the Southern Hemisphere (all land below Earth's equator). Autumnal or Fall Equinox occurs in September for the Northern and in March for the Southern.

What Is a Solstice?
A solstice represents the time when the lengths of day and night are at the greatest variance from each other. It initiates a new season, either summer or winter. Summer Solstice marks the longest day and shortest night-the days thereafter become progressively shorter, the nights longer. Winter Solstice yields the shortest day and longest night-the days thereafter become progressively longer, the nights shorter.

What Causes a Solstice?
Given that the Earth orbits the Sun and that we only perceive the Sun as moving, a solstice occurs when we perceive the Sun to momentarily stand still along its ecliptic path and then to reverse its direction. At the same time, the position of the Earth in its orbit is such that its axis leans toward the Sun, thus situating one of Earth's poles at the closest it ever comes to the Sun-innately, the opposite pole is at its farthest away. Six months later, the reverse is true. The pole that was at its closest becomes the farthest and vice versa.

When are the Solstices?
The two annual solstices occur in June and December for the Northern and Southern Hemispheres. However, they are opposing-summer v. winter. For example, in June, Summer Solstice occurs for the Northern Hemisphere when the North Pole is at its closest to the Sun. Since this is when the South Pole is at its farthest from the Sun, Winter Solstice occurs simultaneously in the Southern Hemisphere. In December, the opposite is true. The North Pole is at its farthest from the Sun while the South Pole is at its closest. Hence, the Northern Hemisphere experiences Winter Solstice. At the same time, the Southern Hemisphere experiences Summer Solstice.

^{*To Convert UTC to Your Time Zone. Using New York City (NYC) as an example, the World Clock website shows during Standard Time the following UTC/GMT† -5 hours. The -5 means that NYC is five hours behind UTC (in other words, offset minus 5 hours). To convert the date and time for the 2012 winter solstice-December 21; 11:12-from UTC to EST (Eastern Standard Time), subtract 5 hours from 11:12. This equals 6:12, which means that the 2012 winter solstice will occur on December 21 at 6:12 a.m. EST.}

^{For a different location, the same process applies. Refer to the number of hours (+ or -) that follow UTC/GMT. Either add or subtract those hours to the UTC time that you want to convert. The World Clock takes into account Daylight Saving Time. It uses the 24-Hour Clock, also known as Military Time. To convert from the 24-Hour Clock to the 12-Hour Clock, click HERE.}

^{† GMT stands for Greenwich Mean Time, the older version of UTC.}

For 2012 dates & times of equinoxes and solstices in the U.S. per Eastern Daylight and Standard Times-as well as dates & times for re-setting clocks, click HERE. For 2011, click HERE.

Note: The Earth with its tilted axis, its distance of 93 million miles from the Sun, and its near-circular orbit around the Sun compose the conditions for its habitability by life as we know it. NASA's Kepler observatory-satellite is searching for other habitable planets. Learn more HERE.

_{^{Sources: wikipedia_equinox; wikipedia_solstice; Solar Physics; nmm Astronomy}}