PacSci Perspectives

Spring Has Sprung

Mar 20, 2019

Spring has officially arrived! March 20 is the Spring Equinox, marking the astronomical first day of spring and the changing of the seasons. Spring is a thriving time in the Pacific Northwest: our cherry blossoms, daffodils and tulips are blooming, and the sun is (sometimes) out!

Speaking of the sun, have you noticed how long our days are? We’re getting over twelve hours of daylight, and as summer approaches, we’ll be getting even more. It’s a misconception that our days are growing longer because the earth is getting closer to the sun–it’s the earth’s tilt that has to do with the seasons changing.

The earth never orbits upright–it has an axial tilt of 23.5 degrees. On the Spring Equinox, also known as the Vernal Equinox, the sun crosses the imaginary line above the earth’s equator from south to north. When the sun passes overhead, the earth’s axis is not directed toward or away from the sun. This means that if you stood on the equator, the sun would be directly over you!

Because of the Spring Equinox, the Northern Hemisphere tilts towards the sun, leading us to have longer and brighter days. On the Fall and Spring Equinoxes, the length of the day and length of the night are very close to equal. The Fall Equinox usually happens around September 20. The equinoxes are also the only days in the year where the sun rises due east and sets due west for everyone on Earth.


Earth's axial tilt.

Earth's rotation around the sun.


So we know that equinoxes occur when the axis of rotation of the earth is parallel to the direction of its motion around the sun, but what are solstices?

Solstices are the days when one of the earth’s hemispheres is pointed towards the sun. They occur when the sun is at its greatest distance from the equator. During the Northern Summer Solstice, which happens around June 20, the sun strikes the Northern Hemisphere as it tilts towards the sun. The Northern Hemisphere receives more solar energy and sunlight than the Southern Hemisphere. The Northern Summer Solstice is the longest day of the year for the Northern Hemisphere.

However, during the Southern Summer Solstice, the sun’s light hits the Southern Hemisphere while the Northern Hemisphere is tilted away from the sun. The Southern Summer Solstice, which is around December 20, is the longest day of the year in the Southern Hemisphere, and the shortest in the Northern.

Starting April 1, come celebrate the changing of the seasons during Spring Fling at Pacific Science Center! Learn more about Spring Equinox and the stars in our sky at The Spring Sky planetarium show, see the symbiotic relationship between butterflies and flowers at our new Living Wall exhibit in our Tropical Butterfly House, and witness the power of pollination with our Fantastic Flora interactive activity. There’s no shortage of spring-themed activities to enjoy.