December Winter Solstice Opens Northern Hemisphere’s Shortest Season

Summary: The 2016 December winter solstice opens the Northern Hemisphere’s shortest season, shortest day and longest night.

solstices and equinoxes as astronomical season placeholders: NASA Science, Public Domain, via NASA Space Place

The December winter solstice, which happens Wednesday, Dec. 21, at 10:44 Coordinated Universal Time (5:44 a.m. Eastern Standard Time), opens the Northern Hemisphere’s shortest season.

The Earth’s north-to-south axial tilt away or toward the sun determines Earth’s seasons. The current tilt of Earth’s polar, or rotational, axis angles at 23.43711 degrees to its perpendicular orbital axis. Earth’s rotational axis always tilts toward the same points in the sky, known as celestial poles, during the Blue Planet’s rotation around the sun. The northern end of the imaginary axis, at Earth’s North Pole, points toward the northern celestial pole. The southern end of the imaginary axis, at the South Pole, points toward the southern celestial pole.

The December solstice marks the axial tilt that favors the Southern Hemisphere with more hours of sunlight than the Northern Hemisphere. The North Pole tilts away from the sun while the South Pole leans toward the sun. A small number of hours of sunlight spells winter for the Northern Hemisphere. A great number of hours of sunlight ushers in summer for the Southern Hemisphere.

Six months later, the June solstice pairs with the December solstice to flip the hemispheres’ seasons. The June solstice favors the Northern Hemisphere with more hours of sunlight than the Southern Hemisphere. The North Pole now leans toward the sun while the South Pole tilts away from the sun. A great number of sunlit hours spells summer for the Northern Hemisphere. A small number of sunlit hours announces winter in the Southern Hemisphere.

A pair of equinoxes combines with the pair of solstices to distinguish Earth’s annual seasons. The March and September equinoxes (Latin: aequus, “equal” + nox, “night”) mark the placement of the subsolar point, where the sun’s rays arrive perpendicularly to the Earth’s surface, exactly over the equator. The solar terminator, which separates Earth’s illuminated, or day, and darkened, or night, sides, aligns perpendicularly with the equator. The Northern and Southern Hemispheres are equally illuminated on Earth’s side facing the sun. The two hemispheres are equally darkened on the side turned away from the sun.

The March equinox is also known as the spring, or vernal, equinox in the Northern Hemisphere and as the autumnal equinox in the Southern Hemisphere. Six months later, the September equinox flips the hemispheres’ seasons. The September equinox announces spring in the Southern Hemisphere and autumn in the Northern Hemisphere.

These four seasonal placeholders endure for varying lengths. Astronomical winter straddles the end of one year and the beginning of the next year. EarthSky lead writer Bruce McClure notes the length of the northern astronomical winter and the southern astronomical summer, running from December solstice to March equinox, as 88.99 days.

The Northern Hemisphere’s astronomical spring, which is the Southern Hemisphere’s astronomical autumn, begins with the March equinox and ends with the June solstice. The timeframe encompasses 92.76 days.

The June solstice signals another astronomical seasonal shift. The northern summer and the southern winter log 93.65 days until the September equinox.

The September equinox announces astronomical autumn in the Northern Hemisphere and astronomical spring in the Southern Hemisphere. The new seasons mark 89.84 days until the December solstice.

The Earth’s orbital closeness to the sun, known as perihelion (Ancient Greek περί, perí, “near” + ἥλιος, hḗlios, “sun”), explains the shortness of astronomical winter. Earth travels faster in proximity to the sun and slower in remoteness from the sun, known as aphelion (Ancient Greek ἀπό, apó, “from” + ἥλιος, hḗlios, “sun”). The December solstice currently precedes perihelion by two weeks. Perihelion in 2017 occurs Wednesday, Jan. 4.

Bruce McClure credits Belgian astronomical algorithm expert Jean Meeus (born Dec. 12, 1928) with identifying the timeframe of 1246 to 6429 for astronomical winter’s status as the Northern Hemisphere’s shortest season. Perihelion coincided with the December solstice in 1246 and will coincide with the March equinox in 6430.

The takeaway for the December winter solstice as opener of the Northern Hemisphere’s shortest season is that Earth’s axial tilt causes astronomical seasons and Earth’s perihelion determines duration of astronomical seasons.

Earth’s remoteness (aphelion) and proximity (perihelion) in orbiting the sun: Alexander Novati, Public Domain, via NASA Jet Propulsion Laboratory SciJinks

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

solstices and equinoxes as astronomical season placeholders: NASA Science, Public Domain, via NASA Space Place @ http://spaceplace.nasa.gov/seasons/en

Earth’s remoteness (aphelion) and proximity (perihelion) in orbiting the sun: Alexander Novati, Public Domain, via NASA Jet Propulsion Laboratory SciJinks @ http://scijinks.jpl.nasa.gov/earths-seasons/

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