Weather Lab: When does the sun rise?

ANCHORAGE, Alaska (KTUU) - Alaska’s Weather Source received a question from a viewer who asked how the is sunrise calculated. The focus of her question was: is it calculated when the sun comes up over the mountains in east Anchorage or on a flat horizon?

The simple answer is, the sunrise is measured based on the flat horizon. There would be no way for someone to calculate where someone might be in the shadows of the mountain.

So if the sunrise is at 4:25 a.m., in Anchorage or other areas with mountains to the east, the sun won’t appear but the sky will brighten that direction.

Because of the mountains, the moment one sees the sun could be well past that sunrise time. In midtown Anchorage, the sun doesn’t make it above the mountains until 20 to 45 minutes after the official sunrise, depending on the time of year.

So why does the sunrise time change, and in Alaska’s case, change so dramatically? It’s mostly because the earth is tilted, by 23.5 degrees. If the earth stood straight up and down on its axis, sunrise and sunset times would never change, but because the earth is tilted, that means one pole — either the north or south — will end up being pointed away from the sun at the edges of the orbit.

As the earth moves around the sun, the tilt doesn’t change direction, so when the north pole is pointed away from the sun and gets no light, that’s winter in the Northern Hemisphere.

As the Alaska slides into summer, the north pole then gets pointed toward the sun and receives more daylight. The changing sunrise and sunset times are the incremental changes of light as the earth orbits around the sun. By the way, this little tilt of the earth is also the reason for seasons.

On the spring and fall equinoxes, one might expect to have equal hours of daylight and darkness. After all, equinox means “equal night,” but because of the way the sunrise and sunset are measured, that doesn’t quite happen.

The sunrise is measured from when the top of the disk appears above the horizon. The sunset is measured when the top of the disk drops below the horizon. So on the equinox, the day is a little longer than 12 hours. If the sunrise and sunset were measured from the center of the disk, then there would be 12 hours of daylight and darkness on the equinoxes.

The day that has 12 hours of daylight and darkness is called the equilux, and that can occur one day to weeks before the equinox.