catalina – When does macOS switch between light and dark mode?

MacOS Catalina added support for automatic light and dark mode switching.

Sample settings screen

The support article linked above reads:

Auto (available in macOS Catalina): Automatically use the light appearance during the day, and the dark appearance at night.

This doesn’t seem to quite capture my experience though, so I’m wondering what the exact algorithm is here.

For example, where I’m currently situated today:

  • in my city, the sun sets 18:46 according to Google
  • in the capital of the country, it sets at 18:43 according to Google
  • my theme switched from light to dark at 18:57

Is this due to simply different services used by Google and Apple for determining the sunset, or is there more to the switching algorithm than meets the eye?