We owe the life of our planet to the Sun. It heats up the earth just right for the water cycle to work. It gives light making plant life possible. And the earth’s atmosphere intelligently keeps the undesirable rays from the Sun’s spectrum from reaching us.
Our scientific, religious, cultural and personal relationships to the sun are multi layered and extremely fascinating. Our system of time and seasons is based on the earth’s motion around the sun.
To understand this, we need to acknowledge the difference between how we perceive things from the earth and how they are really in the space.
From the earth, it seems that we are stationery and the skies move from east to west (the sun, stars, constellations). For almost 1300 years, we believed that the earth was the center of the universe. Ptolemy accepted Aristotles geocentric model (earth at the center and the other planets and the sun wandering around it in a fixed orbit.) and carried it forward. It was only in the 1500s that Copernicus published his paper about the heliocentric model (sun at the center and the earth moving around it).
So to start understanding, lets take the Ptolemy model (with the exception that Ptolemy’s ecliptic was circular and did not consider obliquity. To explain obliquity, however, Ptolemy added an epicycle, an orbit within an orbit, for the wanderers.).
Imagine that the earth is stationery and the sun is moving around the earth.
Since the earth’s north pole is tilted towards the Sun by 23.6 degrees (this is called obliquity and is decreasing at a rate of 1 degree per century) and the direction of the tilt doesnt change, it would look something like this.
The path of the sun around the earth is called the ecliptic and the sphere with ecliptic as the diameter and the earth at its center is the celestial sphere. The sun covers the entire ecliptic in one year (365 days or so) The north/south pole of the earth align with the north/south celestial poles. imagine a huge ball and our earth is at the very centre of this infinitely huge ball we see as the universe.
The diameter of this sphere is of course twice the distance between the earth and the sun but we know that our universe extends much beyond that. It doesnt really matter because we see the sky in 2d (we can only guess the difference in different stars because of their brightness).
When the sun is at A, its angle of declination (angle between the plane of the sun and the celestial equator) is maximum in the negative. It increases to 0 at B and then increases to its maximum value in positive at C. As it crosses C, the angle of declination starts decreasing being 0 at D and then maximum at its negative value again at A.
Now let us consider Copernicus’s heliocentric model or what is common knowledge today.
The obliquity of the earth,
the fact that the direction of obliquity doesnt change with the rotation of the earth and,
that the earth revolves around the sun,
together are responsible for solstices, equinoxes and seasons.
Also because of the obliquity of the earth, the seasons and solar phenomenon are opposite in the northern and the southern hemisphere.
What is a SOLSTICE ?
Solstice is derived from two Latin words: “sol” meaning sun, and “sistere” to cause to stand still. The perception that the “sun stands still” comes from the shadow on the sun dial changing minimally on and near the solstices.
For someone looking from the skies
In its motion along the ecliptic, the sun has maximum declination at two times (positive and negative) in a year. These are called the solstices (Positions A and C).
Astronomically the significance of solstice:
- The dates of maximum tilt of the Earth in relation to the Sun correspond to the Summer Solstice and Winter Solstice. Earths north pole (in case of SUMMER solstice) or south pole (in case of WINTER solstice) is closest to the sun.
- The sun is directly over the Tropic of Cancer (SUMMER solstice for Nothern hemisphere and WINTER solstice for southern hemisphere ) and the Tropic of Capricorn (WINTER solstice for sonrthern hemispshere and SUMMER solstice in southern hemisphere).
- The sun is also at its greatest distance from the equator.
- This is the time when the sun in the ecliptic changes its direction from going higher (WINTER solstice) to going lower (SUMMER solstice) and again higher.
From our perspective from the earth
As the Earth travels around the Sun in its orbit, the north to south position of the Sun changes over the course of the year due to the changing orientation of the Earth’s tilted rotation axes.
If you are in the Northern hemisphere, you are the closest to the sun you can ever be being on earth in a year at a June solstice (SUMMER) and farthest from the sun you can ever be being on earth in a year at a December solstice (WINTER).
If you are in the southern hemisphere, you are the closest to the sun you can ever be being on earth in a year at a December solstice (SUMMER) and farthest from the sun you can ever be being on earth in a year at a June solstice (WINTER).
June Solstice, the northern hemisphere is closer to the sun than the southern hemisphere. Hence, warmer northern hemisphere and cooler southern hemisphere. At December solstice, the southern hemisphere is closer to the sun than the northern hemisphere. Hence warmer southern hemisphere and cooler northern hemisphere.
Midnight Sun and Polar Night
On the June solstice, the midnight sun is visible (weather permitting) throughout the night, in all areas from just south of the Arctic Circle to the North Pole.
On the other side of the planet, south of the Antarctic Circle there’s Polar Night, meaning no Sunlight at all, on the June Solstice.
Summer solstice is the longest day in a year.
Winter solstice is the shortest day in a year.
For someone looking from the skies
The equinox marks the exact moment twice a year when the Earth’s axis is not tilted toward or away from the Sun at all (Positions B and D). However, the axis tilt of around 23.4 degrees toward the celestial pole, remains the same.
From our perspective from the earth
“Equinox” means “equal night” in Latin. But even if the name suggests it and it’s widely accepted, it isn’t entirely true that day and night are exactly the same. 12 hours day and 12 hours night – on the equinox all over the world – only nearly.
Even if day and night aren’t exactly equal on the day of the equinox, there are days when day and night are both very close to 12 hours. However, this date depends on the latitude, and can vary by as much as several weeks from place to place.
On the equator, the day (hours with daylight) and night (hours without daylight) stay approximately the same length all year round.
After the Winter solstice the Sun follows a higher and higher path through the sky each day until it is in the sky for exactly 12 hours. This occurs on the Spring Equinox.
After the Spring Equinox, the Sun still continues to follow a higher and higher path through the sky, with the days growing longer and longer, until it reaches it highest point in the sky on the Summer Solstice. After the summer solstice the Sun follows a lower and lower path through the sky each day until it reaches the point where it is in the sky for exactly 12 hours again. This is the AutumnEquinox.
After the Autumn Equinox the Sun will continue to follow a lower and lower path through the sky and the days will grow shorter and shorter until it reaches its lowest path on the Winter Solstice.
The variation in dates of solstices and equinoxes every year is because of Precession.
Tidal forces from the Sun cause the earth’s axis to wobbles. The north/south pole trace a circle on the celestial sphere as the earth wobbles. This takes 26000 years to complete one wobble/circle.
Next time you look at the sky, look closer.