Daylight Saving Time Through an Astronomer's Eyes
A UArizona astronomer explains the science behind the changing of the seasons and the impetus behind why most states change their clocks twice a year.

By Mikayla Mace Kelley, University Communications
March 12, 2021


Clock with flowers
On Mar. 14, every state - except Hawaii and most of Arizona - will spring their clocks forward an hour, and fall back on Nov. 7.

Daylight saving time begins March 14 at 2 a.m. for most of the country. But while everyone else springs their clocks forward one hour, Hawaii and most of Arizona will be keeping time.

Daylight saving was originally implemented for agricultural and business reasons, to allow for extra hours of daylight in the evening when most people are still occupying their time. Congress passed the Uniform Time Act in 1966, which established three new time zones and start and end dates for daylight saving time. But the law also allowed states to opt out, which Arizona did two years later.

"Daylight saving time is a political thing, not a scientific thing," said Thomas A. Fleming, a University of Arizona astronomer in the Department of Astronomy and Steward Observatory.

However, science is involved in the changing of the seasons, with changing daylight hours caused by the tilt of the Earth as it orbits the sun.

"Earth's axis is not aligned perpendicular to its orbit. Instead, it is tilted 23.5 degrees," Fleming said. "Venus rotates around an axis that runs straight up and down, for example, so it doesn't experience seasons or changes in daylight hours throughout a year."

The Earth's tilt means that for about half the year, the northern hemisphere is tilted toward the sun and during the other half, it's tilted away. On the fall and spring equinoxes, the sun rises exactly in the east and sets exactly in the west.

"Imagine driving west down Speedway (Boulevard in Tucson) in mid-March," Fleming said. "The sun is right in your eyes. But during the summer, the sun rises in the Northeast and sets in the Northwest, resulting in a longer path in the sky and more daylight hours."

In the winter, the sun rises in the southeast and sets in the southwest, which means shorter daylight hours.

People in many places would rather have that extra hour of sun in the evening than the morning, but Fleming said he's grateful that hot Arizona evenings are cut short.

He also added that the farther south you go, the less of a difference that variability in daylight makes.

"I can understand why Hawaii opted out of daylight saving," Fleming said. "There are very few seasonal differences that far south. Alaska is the one I can't figure out. It's light all the time in the summer, so the time doesn't really matter."

Most clocks will fall back in the U.S. on Nov. 7.


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Thomas Fleming

Department of Astronomy and Steward Observatory