# The One and Only Lunar Dial in the Forbidden City

The one and only lunar dial in the Forbidden City is set in front of the Palace of Compassion and Tranquility (Cininggong). It was used to tell the time by measuring the position of the moon in the sky.
 Location Map of Lunar Dial in the Forbidden City

## What is the lunar dial like?

The lunar dial is composed of two concentric plates and a vernier. The smaller, rotatable top plate is called Heaven Plate engraved with 24 hours of a day while the larger, non-rotatable lower one is Earth Plate engraved with 30 days of a lunar month.

The lunar month is measured by the movement of the moon. The day when the moon is completely invisible is called Shuori, which is the first day of the lunar month; the day when the moon is the roundest is called Wangri, which is the 15th (or 16th) day of the lunar month. One cycle from Shuori to next is a lunar month.

## How to use the lunar dial to tell time?

The angle between the lunar dial plates and the ground should be set equal to the latitude where the lunar dial is used. For example, the Forbidden City is located at 39 ° 54, so the angle between the plates and the ground is exactly 39 ° 54. To use it, the Heaven Plate should be rotated according to the rise time and rise angel of the moon, which is indicated on the Earth Plate, and the vernier should point directly to the moon, then read the scale on the Heaven Plate to know the time.

However, the lunar dial can indicate the time accurately only on a full moon night, because the moonrise time is delayed by an average of 48 minutes every day, so the time indicated by the lunar dial after the full moon will be 48 minutes faster on average every day assuming that there is enough moonlight to read the time. Therefore, in the week before the full moon, the time indicated by the lunar dial will differ from the actual time by 5 hours and 36 minutes.

To solve this problem, the more advanced lunar dial includes a chart showing how to calculate to get the right time, and a plate to adjust longitude and latitude.

## Why the one and only lunar dial is in the Palace of Compassion and Tranquility?

As the lunar dial cannot tell time accurately most of the time, the one in the Palace of Compassion and Tranquility is more importantly used to show the Empress Dowager's supreme position among all concubines as the Palace of Compassion and Tranquility is the empress dowagers’ residence.

And ancient Chinese believed that the male dominates the Yang and the female dominates the Yin. The former is masculine and positive while the latter is feminine and negative. The moon stands for the Yin so the lunar dial is put in front of the highest ranked female’s residence.

Actually, as Empress Dowager gave birth to the emperor, the highest ranked male, a sundial standing for Yang is also set in front of the Palace of Compassion and Tranquility, symmetrically to the lunar dial along the central axis of the palace.