Compare sunset from two heights
Record the horizon crossing from two measured heights at one location. A higher observer can retain a view after the lower observer loses it.
What you need
- Two observers or synchronized fixed cameras
- Measured heights above the same horizon surface
- UTC-synchronized clocks and a safe solar recording setup
Collect the readings
- Choose an unobstructed horizon and place both observers before sunset. Use equipment designed for safe solar observation; never look through an unfiltered telescope or camera viewfinder at the Sun.
- Record camera heights, coordinates, weather and whether the timed contact uses the Sun’s centre, upper edge or lower edge. Both observers must use the same contact convention.
- Time the same contact from both positions. Repeat with the observers or cameras swapped when possible. Keep clock uncertainty separate from the uncertainty caused by a blurred horizon.
- Compare horizon dips from the heights, then evaluate the Sun’s vertical motion at that date and location. Near high-latitude shallow crossings, a small dip difference can correspond to a larger time difference.
Compare the predictions
A higher observer has a lower geometric horizon.
The Sun’s crossing speed depends on its path and the location, so one universal seconds-per-metre rule is unsuitable.
Refraction and local terrain can shift both contacts.
Open the related calculation →