Wednesday, December 20, 2017

Winter Solstice 2017: The First Day Of Winter

Winter Solstice 2017

The Winter Solstice falls on Thursday, December 21, 2017 at 11:28 a.m. Eastern Time (4:28 p.m. Universal Time), according to the U.S. Naval Observatory.

What is Winter Solstice 2017?

The Winter Solstice, as pictured in the above NASA image, is caused by a tilt of the earth's rotating axis and marks the first day of winter. It is the shortest day and longest night of the year in the northern hemisphere for locations like Melbourne, Florida, but the reverse happens in the southern hemisphere for locations such as Melbourne, Australia.

The Winter Solstice can occur on December 20, 21, 22, or 23, depending on calendar events such as leap year and when the Solstice begins relative to Coordinated Universal Time.

According to NASA, it is not the Sun that is moving north or south through the seasons, but a change in the orientation and angles between the Earth and its nearest star. The axis of the Earth is tilted 23.5 degrees relative to the Sun and the ecliptic plane. The axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly.

But how can it be called the First Day of Winter 2017 and Mid-Winter at the same time?

Although the December Solstice marks the beginning of Northern Winter, it is often called Mid-Winter. The difference lies in the definitions created by culture, agriculture and astronomy. According to astronomers, December 21st marks the beginning of Winter in the Northern Hemisphere and the beginning of Summer in the Southern Hemisphere.

The official start and end of Winter can vary by country - not because Winter starts a week or so earlier in one country than another in the same hemisphere, but because the recognition of the start of Winter is often influenced by historical or cultural reasons particular to that country.  Most countries recognize Winter as starting on dates ranging in November and ending sometime in March.

Why does there seem to be such a lag time between the longest night and the coldest days?

Blame the oceans, which heat up and cool down slowly.   Although the Winter Solstice marks the lowest exposure of the northern Atlantic and Pacific Oceans to the Sun's heating radiation, the oceans are still warm in the Northern Hemisphere from the summertime, and that delays the peak heat by about a month and a half.

Winter Solstice Sunlight On Earth
Winter Solstice Heating Radiation On Earth. Credit: NASA

Similarly, in June the water still cold from the Winter, and the average warmest days are still a month and a half ahead.

Summer Solstice Sun Energy On Earth
Summer Solstice Heating Radiation On Earth. Credit: NASA