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Solar Time

The moment of the Sun’s zenith occurs simultaneously in all points lying on the same meridian. The Sun reaches its zenith at different heights, but everywhere it reaches the highest altitude during that part of the day. The time that passes between successive Sun shines, is called a day. The Earth is doing one revolution around the axis in this time. It was assumed that at the moment it reaches its zenith, it is exactly 12:00 pm solar timesolar timesolar time. The whole day lasts 12 hours before this and 12 hours after this moment. The whole day is therefore divided into 24 hours. Knowing the rotation time of the Earth and knowing that every point on it turns a circle, you can calculate at what rate the Earth rotates.

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Na rysunku widać tarczę słoneczną. Pod nią znajduje się zielone pole oznaczające powierzchnię Ziemi. Słońce znajduje się na strzałce wskazującej kierunek pozornego ruchu względem Ziemi - The apparent movement of the sun - ze wschodu na zachód. Na polu znajdują się linie oznaczające kolejne południki z zaznaczonymi szerokościami geograficznymi. Różnice te wynoszą jeden stopień. Południk, nad którym znajduje się Słońce, ma godzinę 12.00, gdyż tam Słońce góruje. Południki na wschód od niego mają godzinę późniejszą, a leżące na zachód wcześniejszą. Różnice w czasie między południkami wynoszą cztery minuty. Na ilustracji nad zieloną strzałką napis: The apparent movement of the sun.

360° : 24 hours = 15°
One hour is 60 minutes. Let us now calculate how many times the Earth will turn by 1°.
60 minutes : 15° = 4 minutes

And what is the difference between time on different meridians? Let's take the difference between the two meridians, 12⁰E and 26⁰E, for example. We know that on the 12⁰E meridian it is 12:20. First, calculate the difference in longitudelongitudelongitude. It should be taken into account that both places lie on one hemisphere. In this case, subtract the values.
26⁰E - 12⁰E = 14⁰
So we know that the meridians are 14 degrees apart. We also know that the Earth rotates at the rate of 1 degree in 4 minutes. So we can calculate the difference in solar time.
14⁰ x 4 minutes = 56 minutes
The meridian 26⁰E lies east of 12⁰E. Therefore, add the result to the known time.
12.20 + 56 minutes = 13:16
On the meridian 26⁰ it is 13:16.

And what if the meridians are in different hemispheres? Let's assume that on our meridian 12⁰E it is still 12:20. We want to find out what time it is on the 24⁰W meridian. If the meridians are in different hemispheres, to calculate the local time, add their values.
12⁰E + 24⁰W = 36⁰
Now we calculate the difference in time:
36⁰ x 4 minutes = 154 minutes (2 hours and 34 minutes).
We now calculate local time. The 24⁰W meridian lies west of 12⁰E – in which case we have to deduct time
12.20 – 2 h 34 min = 9:46

On the meridian 12⁰E it is still 12:20. Let's now calculate where the meridian is midday at the moment. If in 4 minutes the Earth rotates by 1⁰ and the time difference is 20 minutes, then we can calculate the meridian we are looking for.
20 minutes : 4⁰ = 5⁰
The meridian is located 5 ° further. But in which direction? Since we have an hour later, we have to head west.
12⁰E - 5⁰ = 7⁰E
Midday is now on the meridian 7⁰E.

Summary

• When the Sun reaches its zenith it means noon in solar time.

• The day lasts 24 hours.

• 1⁰ Longitude corresponds to 4 minutes of solar time.

Keywords

solar time, day, longitude

Glossary