When day equals night

Astronomy

Today happens to be the autumnal equinox, a minor astronomical event. March 21 is the so-called vernal equinox. What exactly are equinoxes? On these days, all over the globe, days and nights are of almost equal duration, i.e. 12 hours each. The earth’s axis of rotation, tilted at 23.5 degrees, points sideways relative to the sun. The sun is thus positioned directly over the equator. Equinoxes occur when the sun’s apparent path across the sky (ecliptic) intersects the celestial equator. This path is inclined at 23.5 to the celestial equator. After the vernal equinox (also called first point of Aries) the North Pole points progressively sunward making the days to grow longer signalling the onset of spring and summer.

The summer solstice (around June 21) is when the sun reaches its highest point of the sky (lowest in the sky for those at the equator) implying that the day is longest on this date. The Latin word for sun and stop gives the name solstice, i.e. sun literally stops moving north that day. (Equinox is equal night and day). The summer solstice sun hovers straight over the Tropic of Cancer. Ancient Greeks noted that the sun was located in the constellation Cancer at the summer solstice. Owing to the precession of the earth’s axis, this shifted to Gemini around the beginning of the Christian era and has now shifted to Taurus (where it will remain for another 2000 years).

After June 21, the days get shorter and at the autumnal equinox again, the day and night are equal. After this, the North Pole points away from the sun and the nights get longer (in the Northern Hemisphere) until on December 22, we have the winter solstice (the longest night in the northern hemisphere) when the sun hovers straight over the Tropic of Capricorn and stops moving south. After the winter solstice, the nights get shorter. In ancient times, this was marked by the feast of the undying sun. They were worried that the sun might keep on going down south and disappear!

After the winter solstice, the days get longer till the vernal equinox (March 21) when they again become equal. The days start getting longer which was why the vernal equinox was a festive occasion, marking the start of spring. The Egyptians built the Great Sphinx to point towards the rising sun on this day. In the southern hemisphere all the events are reversed. After the autumnal equinox, the South Pole points increasingly sunward, signalling the onset of spring and summer. But because most ancient civilisations (with recorded history) were in the northern hemisphere, these astronomical events were described from this perspective.

Equinoxes on other planets

What about equinoxes on other planets in our solar system? It all depends on how they tilt. The axis of Mars is tilted at about 25 degrees, almost similar to that of the earth.

Martian seasons are comparable to terrestrial ones, except that they are about twice as long in duration. Mars has an orbital period of 687 days and its vernal equinox is around Jan 22, when the Northern ice cap is more extended, as winter would have just ended in the arean northern hemisphere.

The equinoxes on Saturn (tilt around 29 degrees) would resemble those on earth. But Saturn’s orbital period is almost 20 years and sunlight intensity at that distance is only one per cent of that falling on earth! This is why the Cassini Spacecraft was not powered by solar energy but had to carry radioisotope thermoelectric generators (RTG). As Saturn emits over twice the energy it gets from the sun, its weather is driven by its own internal energy sources of heat. The seasons are similar to earth.

The curious case of Uranus

Uranus is especially odd, as its poles are tilted 98 degrees relative to its orbit, so that the planet is lying on its side as it goes around the sun. The planet’s South Pole pointed directly sunward when the Voyager 2 Spacecraft passed by Uranus in 1986. This meant that the southern hemisphere of the planet was midway in the 21-year-long summer of continuous illumination while the higher northern latitudes was in stygian darkness for the same duration. The orbital period of Uranus is 84 years, so each season would last 21 years. The seasons in Neptune are similar, but being much further away, sunlight plays a minor role.

Mercury has its poles pointing almost straight up and is the only planet with a permanent equinox. The poles always remain on the nocturnal edge, allowing ice to survive in shadowed craters, although it has the highest sunlight intensity. A similar small tilt angle (away from vertical) causes ice to accumulate in shadowed lunar craters!

The tilt of Venus is similar to Mercury, hardly 20, so that Cytherean seasons despite the thick atmosphere would not be prominent. The thick atmosphere of Venus (100 times the earth’s) would distribute radiation uniformly over the planet.

Jupiter has about the same tilt as Venus and its seasons would not be prominently distinct. On the contrary Pluto has a nearly 60-degree tilt.

One or the other of its equinoxes coincides when Pluto is at perihelion. This coincidence results in the warming of its frozen surface, giving it a temporary thin atmosphere. After the last equinox in 1987, more and more of Pluto’s southern hemisphere is getting blotted into darkness which will last over a century, as Pluto’s orbital period is 248 years.

A season on Pluto would last for around 62 years. This is the reason why the New Horizon Spacecraft has been “rushed” to arrive at Pluto by 2016, before the atmosphere condenses back on the surface as the planet is receding further away from the sun.