PORTLAND, Ore. (KOIN) – Monday is the first day of spring, which means it’s the vernal equinox – a time when both the north and south poles of the earth are equal distances from the sun.
Spring officially begins in the Pacific Time Zone at 2:24 p.m. At this point, the sun will be directly over Earth’s equator before crossing over from south to north.
In Portland, the noon sun will reach its mid-point in the sky near 45 degrees from the southern horizon on March 20, according to Jim Todd, director of space science education at Oregon Museum of Science and Industry.
“On the day of an equinox, it is a good day for finding due east and due west from your own backyard,” Todd said. “Just go outside around sunset or sunrise and notice the location of the sun on the horizon with respect to familiar landmarks.”
The Northern Hemisphere receives the sun’s rays most directly in the summer because of the Earth’s 23.4-degree axis tilt. In the winter, the Northern Hemisphere is tilted away from the sun, causing the sun’s rays to pass through the atmosphere at a greater slant, resulting in lower temperatures.
In the summer, the Northern Hemisphere is tilted closer to the sun, resulting in warmer weather. Vernal equinox marks the start of the Northern Hemisphere receiving more direct sun in preparation for the summer season. It is also the start of autumn in the Southern Hemisphere.
During both the spring and fall equinoxes, both hemispheres receive nearly equal amounts of daylight.
In Portland, March 17 – not March 20 – is the day when day and night both last about 12 hours. The sun rises at 7:18 a.m. and sets at 7:19 p.m.
“At the 45th latitude North, the time it takes for the sun to fully rise and set, which is several minutes, is added to the day and subtracted from the night, and therefore the equinox day lasts a little longer than 12 hours,” Todd said.
He said another reason why the day is longer than 12 hours on an equinox is because the Earth’s atmosphere refracts sunlight. After March 17, the daytime will be longer than the nighttime until September 25, after the autumnal equinox.