धूपघड़ी और पृथ्वी की धुरी , Why gnomon of Sundial is parallel to Earth's Axis
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- Опубликовано: 6 фев 2025
- Let us look at the design of equatorial sundial.
This is an equatorial sundial.
Sundial is one of the oldest known instruments for telling time during the day.
This stick is called a gnomon or sanku or stick.
It casts a shadow onto the dial. As the sun moves across the sky, the shadow of the gnomon aligns with different hour-lines on the dial, indicating the time of day.
The gnomon must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. In the Northern hemisphere, it normally points to the pole star Polaris.
Let us understand why ?
Until medieval times, the time was told by 12 unequal hours between sunrise and sunset.
So 1 hour could be 65 minutes long or it could be 55 minutes long depending on the season.
Thus the daylight hours were longer in summer than in winter.
However, the average duration of the day was 24 hours for the whole year.
Astronomers discovered that, using a gnomon parallel to the earth’s axis, a sundial could be made that told equal length hours throughout the year.
Let us try to understand the design of an equatorial sundial.
We know that Earth revolves around the Sun.
Earth also rotates on an imaginary axis around itself as it revolves.
This imaginary line is an equator which divides earth into two hemispheres.
Northern Hemisphere and Southern Hemisphere.
Let us remove the Northern hemisphere and insert a disc.
This disc can be considered as an extension to the Earth’s equator.
Relationship between Earth’s axis and equator is the basis for the equatorial sundial.
Visualize Earth’s axis as a rod and this disc as the slice of the earth taken at equator.
Rod becomes a shadow casting device and disc becomes a face on which shadow is cast much like a clock face.
This one full circle is 360 degrees.
We will divide this disc into 24 equal parts. Each division is 15 degrees.
We will draw these hours on both sides of the disc.
Each 15 degrees of longitude is equal to 1 hour.
Same can also be visualized with the help of this 3D printed component.
These 24 edges represent imaginary longitude lines spaced at 15 degrees.
Let us keep the dial on the ground with a rod or gnomon pointing upwards.
If we observe the passage of shadow cast by gnomon, it is not uniform.
We can rearrange the disc such that the pointer is now parallel to earths’ axis, that is pointing towards the pole star.
Now the shadow moves uniformly and covers each segment in 1 hour.
But what about the orientation of the stick or gnomon ?
Let us use this big globe again.
We will place one dial at the North Pole.
Rotating the globe uniformly, the shadow moves one segment at the rate of 1 hour.
Small dials can be placed at different locations along the same longitude. In this case 75 E.
Sun is overhead around noon at all these locations.
But as you can see, clocks do not match. Dials are indicating different time.
Rod or gnomon at these locations is not parallel to the Earth’s axis.
In order to get the correct time, we have to align the small sundial properly.
With the help of this attachment, let us keep the dial parallel to the equator and gnomon parallel to the axis of the earth.
Now it is matching. Same is true for other locations as well.
At all locations, the gnomon is parallel to the Earth’s axis and the disc is parallel to the equator.
If we set this dial to the North Pole, it will look like this with the gnomon straight up.
This is how it will look if we place the sundial at the equator, that is zero degree latitude.
Anywhere between equator and pole, we have to tip the dial to match the angle equal to that of our latitude.
This is how it will look from the side.
Now you know why we have to always orient the gnomon towards the pole star.
As you have seen , time told by the sundial and that of our clock does not match many times.
That is something which we will discuss in the next video !
Thank You.
#sundialgrowers
#astronomy
#time
Amazing explanation
बहुत बढ़िया 👏
Master piece
Amazing explanation and demo. 👍👍👍
Very informative ! Gnomon ka tip ka shadow, poore din mei, dial pe jo path trace karta hei wo ek Hyperbola hota hei aisa kisi video mei dekha tha... par uska reason kya hei wo nahi samaj paya ... Ho sakei to thoda prakash is baat pe daale. Dhanywaad 🙏
❤❤❤❤
main apka video kafi dekta hoon ,kafi gyanbardhar aur informative hota hai.
३३,अंश धरि घूमिए पृथ्वी विजय चेतफल व्यास पृथ्वी
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