Researcher from the Center for Science and Space at the National Institute of Aeronautics and Space (LAPAN), Andi Pangerang explained two factors that affect the time of sunrise from the sun.
The first factor is from the apparent motion of the Sun, and the second is from the value of one of the parameters to determine time which is called the equation of time.
Andi explained that astronomically the Sun would rise earlier for the southern hemisphere.
It occurs when the southern axis of Earth’s rotation is closer to the Sun, namely at the December solstice, which will occur every 22 December.
While in the northern hemisphere, the sun will rise late because the northern axis of rotation of the Earth is away from the sun.
“On the contrary, during the June solstice, the northern hemisphere or the northern hemisphere’s axis of rotation will tilt toward the sun so that the sun will rise faster, while in the southern hemisphere its rotational axis will move away so that the sun will rise later,” said Andi when contacted by CNNIndonesia.com, Tuesday. (2/11).
Timer
For the second factor, which is related to the time averaging parameter, Andi explained that this time averaging is actually the difference between the true hour shown by the sun’s shadow and the average clock.
The difference between the actual hour and the average hour is also influenced by two factors.
First, the curvature of the Earth’s orbit which is not completely zero, but 1/60, meaning that the Earth will be at the closest point to the Sun or also called the perihelion point and Earth is also sometimes at the farthest point from the sun or called aphelion.
“When the Earth moves from perihelion to aphelion, time will feel slower. Why? Because the speed of the Earth’s revolution is decreasing, while from aphelion to perihelion it means that the value of the speed of the earth’s revolution will be greater so that time will feel faster,” said Andi.
Earth’s axis tilt
The second factor is the tilt of the Earth’s axis. According to Andi, the tilt of the Earth’s axis also varies which is expressed as the value of the sun’s declination, namely the tilt of the axis relative to the celestial pole or the relative position of the sun measured from the equator.
At the March Equinox, the Sun’s declination value is zero, meaning that the Earth’s axis of rotation will be perpendicular to the Sun, so no hemisphere of the Earth is tilted more towards the Sun, either north or south.
At the solstice where the Earth’s axis will be tilted towards the sun in a certain hemisphere, it produces a positive solar declination value.
Meanwhile, when the other hemisphere is far from the Sun, the Sun’s declination value is the most negative.
“What is the impact on Grader this time? So when the Earth’s axis of rotation moves from the solstice to the equinox the time will be slower. Meanwhile, from the equinox to the solstice, time will accelerate,” said Andi.
The combination of these two factors, either the curvature of the Earth’s orbit or the tilt of the Earth’s axis, is what causes the sun to rise faster when measured using an average sundial. The peak itself is around the beginning of November.
Average hours and true hours
Furthermore, Andi also explained that the clock we use (WIB, WIT, WITA) is the average sundial because it refers to the media or the average local first longitude, so this sundial assumes that the time of day will remain 24 hours.
However, the middle time does not always fall at 12.00 but varies between 11.44 to 12.14.
“Meanwhile, if we use a benchmark with a true sundial, namely the hour pointed by the shadow of the sun, then noon will always show 12.00,” said Andi.***
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