The factors that affect the timing effect of the sundial in the Forbidden City in Beijing are as follows

1. Weather factors: Cloudy days, rainy days, foggy days and other meteorological influences will affect the time service effect of the sundial, because the sundial relies on direct sunlight for time service.

2. Seasonal factors: the sun's height is lower in winter and higher in summer, which will also affect the timing effect of the sundial.

3. Observation angle factor: The observer's observation angle will also affect the timing effect of the sundial. If the line of sight is not in the correct position, the sundial timing will also have errors.

4. sundial material factor: the material used in sundial will also affect the timing accuracy. Different materials have different reflectivity and refractive index for direct sunlight, which will affect the timing accuracy of sundials.

5. Geographical location factor: The latitude and longitude of different parts of the earth are different, and the solar altitude angle is also different, which will also affect the timing accuracy of the sundial.

6. Monitoring method: The method of monitoring the sundial will also affect the timing effect. For example, using empirical methods to calculate judgments will have different results from using instruments to measure records.