Vernal equinox: How to see spring begin, just by looking at the stars

Spring is coming to the Northern Hemisphere, but how can you tell when it will start? No need to Google the vernal equinox; to work out when the season officially begins, you only need to look at the stars.

The beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere is marked by the vernal equinox — the point when Earth's axis is side-on to the sun, spreading light and heat equally around the globe. This year it occurs on March 20.

This moment in Earth's annual journey around the sun marks the end of one season and the beginning of the next, and stargazers can spot the change because the springtime night sky, as seen from the Northern Hemisphere, is full of stars and constellations only ever seen at this time of the year.

The easiest way for stargazers to observe this seasonal transition on the equinox is to look southwest after sunset and bid farewell to the bright stars of winter. Find the three unmistakable stars in Orion's Belt — Alnitak, Alnilam and Mintaka — and begin to identify 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, while to its left and slightly below is Sirius, the brightest star in the night sky. Use these three stars to form an equilateral shape — the Winter Triangle.

Related: March 29 solar eclipse: Where and when to see the rare sunrise solar eclipse from North America

For celestial confirmation of spring, gaze north, and you'll see the seven bright stars of the Big Dipper/Plough riding high in the sky — as in the memory aid, "spring up, fall down." As this asterism climbs higher in the sky, use its stars to find the Spring Triangle: an equilateral triangle made of the springtime stars Arcturus, Spica, and Denebola. All three stars are visible with the naked eye, but a good backyard telescope or pair of stargazing binoculars can enhance the view.

Trace the curve of the Big Dipper's handle to "arc to Arcturus," identifying a bright reddish star above due east. Now "spike to Spica," traveling a short distance to identify that bluish star rising in the east-southeast. Directly above it will be Denebola in Leo, a classic spring constellation. (Some skywatching guides may replace Denebola with Regulus, another star in Leo that appears slightly further east near the lion’s front paw; Regulus is a brighter star, but using it as a waymarker stretches out the Spring Triangle considerably).

The Spring Triangle will remain visible in the night sky until August, when summer begins shifting into fall.