The constellation Leo includes one of the three stars of the Vernal Triangle, an asterism that occurs over several months beginning with the spring equinox. (Image credit: manpuku7 via Getty Images)
Spring is coming to the Northern Hemisphere, but how do you know when it's starting? Don't bother searching for information about the vernal equinox online; just look at the stars to find out when the season officially starts.
The start of spring in the Northern Hemisphere and autumn in the Southern Hemisphere is determined by the vernal equinox, the moment when the Earth's axis is oriented sideways to the Sun, distributing light and heat evenly across the planet. This year, that event will occur on March 20.
This moment in the Earth's annual cycle around the Sun symbolizes the end of one season and the beginning of the next, and astronomers can notice these changes because the spring night sky, viewed from the Northern Hemisphere, is filled with stars and constellations that are visible only at this time of year.
An easy way for astronomers to observe this seasonal transition to the equinox is to look to the southwest after sunset and bid farewell to the bright stars of winter. Find three unmistakable stars in Orion's Belt — Alnitak, Alnilam, and Mintaka — and begin identifying the bright stars around them.
Above the belt is Betelgeuse, one of the brightest stars in the night sky, which appears slightly reddish to the naked eye if you squint. Below the belt is Rigel, and slightly to the left and below is Sirius, the brightest star in the night sky. Use these three stars to form an equilateral figure, the Winter Triangle.
For celestial confirmation of spring, look north and you’ll see the seven bright stars of the Big Dipper/Plough rising in the sky — just like the “rising, falling” clue. As this asterism climbs higher in the sky, use its stars to locate the Vernal Triangle: an equilateral triangle formed by the spring stars Arcturus, Spica, and Denebola. All three stars are visible to the naked eye, but a good backyard telescope or a pair of binoculars can improve viewing.
Follow the curve of the Big Dipper's handle to the “arc of Arcturus,” identifying a bright, reddish star due east. Now “steer to Spica,” a short distance, to find this bluish star rising in the east-southeast. Directly above it will be Denebola in Leo, the classic spring constellation. (Some astronomy references may replace Denebola with Regulus, another star in Leo that appears slightly to the east, near the lion's foreleg; Regulus is a brighter star, but using it as a reference point greatly increases the size of the Spring Triangle.)
The Vernal Triangle will be visible in the night sky until August, when summer begins to give way to fall.
Jamie CarterNavigate Social LinksLive Science Contributor
Jamie Carter is a freelance journalist and regular Live Science contributor based in Cardiff, UK. He is the author of A Stargazing Program For Beginners and lectures on astronomy and the natural world. Jamie writes regularly for Space.com, TechRadar.com, Forbes Science, BBC Wildlife magazine, and Scientific American, among others. He edits WhenIsTheNextEclipse.com.
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