What are the Seven Tracks in Chinese Astronomy?
The Sun Dial is a very simple instrument. It consists of a flat
horizontal surface with a vertical post at its center.
The Chinese astronomers have used it to study astronomy with remarkable results.
Time of Day
Each day, as the sun rises in the morning and sets in the evening, the
shadow of the vertical pole (called the gnomon) moves in a circular arc
on the ground. By marking the position of the shadow, one can tell the
time of the day in local time. With proper constrction, an accuracy of
one minute can be achieved.
Sun Dials were used to tell time for centuries. It wasn't until
a few hundred years ago that mechanical watches were invented.
Day of the Year
Each day the tip of the shadow tranverses an arc on the ground.
On each successive day, the arc is different, because the position of the
Sun is different on the sky.
In the Summer, the Sun is high in the sky. The shadow is short. The arc
has a short radius and is closest to the post.
In the Winter, the Sun is low in the sky. The shadow is long. The arc
has a long radius and is farther away from the post
It takes 6 months to go from the longest day in the Summer to the
shortest day in the Winter. And another 6 months to go from the
shortest day back to the longest day in the Summer.
The Chinese astronomers mark 7 tracks and calls them "heng"
. They are one-month apart.
The innermost track marks the day of Winter Solstice and the
outermost track marks the day of Summer Solstice.
Such a Sun Dial still exists in the Beijing Observatory.
This is described precisely and in detail in Zhoubi Shuijing
Volume 1, Chapter 3 as follows:
Precise observations were made which determined the length of year to be 365 and 1/4 days long,
as stated in the above reference.
Sun Dial Page