Jimmy Westlake: Spring arrives Friday | SteamboatToday.com
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Jimmy Westlake: Spring arrives Friday

The season of spring officially arrives in the Northern Hemisphere Friday at 3:45 pm, Colorado time. That’s the moment when the sun crosses the equator on its way north — what we call the vernal equinox.

With each passing day, the sun rises a little bit earlier and sets a little bit later, increasing the number of daylight hours that we enjoy. Our daylight hours will continue to increase until the summer solstice at 9:38 a.m. June 21, the longest day of the year and the first official day of summer in the northern hemisphere.

Thanks to the 23.5 degree tilt of the Earth’s axis, the sun spends half of the year shining down on the northern hemisphere and the other half of the year shining down on the southern hemisphere. Separating these extremes are two days called the equinoxes, six months apart, when the sun shines straight down on the midpoint, the Earth’s equator.



Equinox is a word that means “equal night” and is used to describe these two special days when every location on Earth experiences exactly 12 hours of sunlight and 12 hours of darkness. The other equinox in 2015, the autumnal equinox, will occur at 1:20 a.m. Sept. 23, when the sun again crosses the equator, but this time heading south.

The closer you are to one of Earth’s poles, the more noticeable are the seasonal changes in day and night. The poles take turns having six months of sunlight followed by six months of darkness. This Friday, Earth’s North Pole will experience its first sunrise in six months, and the South Pole will experience its first sunset in six months.



Do other planets experience seasons the way that Earth does? Indeed, they do.

Take the planet Mars, for example, whose 25-degree tilt on its axis is just slightly greater than Earth’s 23.5-degree tilt. The similar tilt makes the Martian seasons very similar to ours, however, each season lasts for twice as long because Mars requires nearly two Earth years to orbit the sun.

Mercury, the innermost planet, spins with its axis nearly straight up and down, so it experiences almost no seasonal variations in its short 88-day year. Uranus, on the other hand, has the most extreme seasons of all the planets in our solar system. It is tilted 98 degrees on its side so that it rolls around the sun more like a bowling ball than a spinning top. Combine that with its 84-year orbit around the sun, and it means Uranus’ north pole receives 42 years of constant sunlight followed by 42 years of total darkness.

For Jews and Christians, the celebrations of Passover and Easter, respectively, are tied directly to the date of the vernal equinox. Passover is celebrated during the first full moon of spring, and Easter is celebrated on the Sunday that immediately follows the first full moon of spring.

This year, the first full moon after the vernal equinox happens on Saturday, April 4, making Sunday, April 5 the date of Easter in 2015.

As a special Passover and Easter bonus this year, the full Egg Moon will be totally eclipsed by the Earth’s shadow before sunrise April 4.

Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in the Steamboat Today newspaper and his “Cosmic Moment” radio spots can be heard on local radio station KFMU. Check out Jimmy’s astrophotography website at http://www.jwestlake.com.


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