Tomorrow, Tuesday, December 22nd, is the Winter Solstice for this year. You may have heard this term used quite often, perhaps in newscasts, newspapers or magazines. Just what is the Winter Solstice and, the question may occur to you, are there analogs to this phenomenon for the other seasons. Well it turns out that there is; more on that below.
The Winter Solstice, in a practical sense, is the shortest day of the year with the longest night. This means that tonight, Monday night, 12/21/2015, is the longest night of the year and that, logically, tomorrow, the actual “Solstice”, is the shortest day.
What does all this mean and why? The earth orbits the sun at a distance of 150 million kilometers in a mildly elliptical orbit so, you might expect that this, the slightly changing earth-sun distance, would be the main driver of the seasons, the annual changing of the earth’s climate; it isn’t. In fact, the earth’s perihelion or the closest point in the earth’s yearly journey about the sun this year occurs on January 4th 2016 or in about 2 weeks, the coldest time of the year; the earth’s aphelion, or the furthest distance from the sun occurs on July 6th 2016, the warmest time of the year for the Northern hemisphere. So what is the main driver of the seasons? The “Angle of Insolation” or the incidence angle of Solar Irradiance or, in other words, how high the sun is above the southern horizon (or northern horizon for our friends in the Southern hemisphere). If the earth wasn’t tilted, the only determinant of the temperature would be the latitude; the closer you are to the equator, the warmer the climate. The earth’s axis is tilted 23.5º to the vertical so, as a consequence, the sun will change in altitude a total of 47º from it’s highest point to its lowest over the course of a year. Tomorrow, the Winter Solstice is that day, the point when the sun is 23.5º below the celestial equator (illustration via Stellarium to the left), the projection of the earth’s equator onto the sky. How does this translate in terms of the sun’s elevation and position in the sky?
For northern hemisphere observers, the altitude of the sun above the southern horizon is the complimentary angle of the observers latitude on the equinoxes, i.e.: for an observer at 40º North, the sun would be 50º above the southern horizon on the equinoxes. This occurs because the celestial equator is 90º from the pole: 180º-(90º+40º)=50º; 180º separates the northern horizon from the southern. Regardless of the date, this relationship is always in play and we can thus compute the elevation of the sun at any date. Another example would be the sun’s elevation at the solstices; at the summer solstice, the elevation above the horizon would be: 180º-(90º+40º)+23.5º=73.5º; at the winter solstice, the elevation would be: 180º-(90º+40º)-23.5º=25.5º. This is the elevation of the sun above the southern horizon, tomorrow, for an observer at 40º north and, as stated previously, 73.5º-25.5º or 47º south of it’s northernmost point reached at the Summer Solstice. This translates into an average daily altitude change of 47º/(365/2) or 0.26º, about 1/2 the width of the full moon every day.
Lets briefly describe the other three major seasonal markers; in total, there are four: 1) The Vernal Equinox, the astronomical beginning of spring; it is the point on the ecliptic where the sun crosses the celestial equator; the first of two days where there are equal hours of daylight and darkness; 2) the Summer Solstice, the longest day and shortest night of the year; 3) the Autumnal Equinox, the second of two days where there are equal hours of daylight and darkness and finally, today, 4) the Winter Solstice, the shortest day and longest night of the year. Suppose you lived at the poles, what would it be like as the seasons change? Lets start with the Vernal Equinox, (March 20th, 2016). If you were an observer at the North or South pole on this day, the sun would be on the horizon for 24 hours, moving along the horizon in a very slight upward spiral if you’re at the North Pole or downward, if you’re at the South Pole. This phenomenon would be reversed at the opposite pole. The circumference of this great spiral would be the entire 360º of the sky! At the North Pole, the upward spiral would culminate at it’s highest point of 23.5º above the horizon on June 21st, the Summer Solstice, regardless of the direction of observation! At the South Pole, the slight downward spiral would culminate at 23.5º below the horizon on the same day, the Winter Solstice for the Southern Hemisphere. The Autumnal Equinox occurs on September 20-21st of each year and what was observed on the Vernal Equinox would occur in reverse order for our observer at the south pole. Consider this, the sun would move along the horizon for 24 hours on the Vernal and Autumnal Equinoxes for both observers on both days!
If the earth’s axis wasn’t tilted, things would be quite boring with the sun rising and setting at the same point every day for the entire year; the only variable really determining climate then would be the observer’s latitude (their point north or south of the equator). History is rich with examples of ancient peoples who’ve understood and celebrated the seasonal markers. As far back as the Neolithic age, with the construction of Stonehenge, the relationship between the sun’s position and the changing seasons was well understood. Other examples include Newgrange, another Neolithic monument, built in the Boyne Valley, County Meath, Ireland to mark the passage of the Winter Solstice.
Coming soon will be my annual presentation of the Winter Skies!