Celestial News: Of equinoxes and aurorae
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, 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 sun up and 12 hours of sun down.
The season of spring officially arrives in the Northern Hemisphere at 2:25 p.m. March 20. 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 up until the summer solstice at 7:58 a.m. June 21, the first official day of summer in the Northern Hemisphere.
The other equinox in 2023, the autumnal equinox, will occur on at 11:50 p.m. Sept. 22, when the sun again crosses the equator, but this time heading south.
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 and Passover falls on April 5, making April 9 the date of Easter.
Another phenomenon that has a connection to the equinoxes is the aurora borealis, or northern lights. We generally get to see aurorae as far south as Colorado only a few times a decade, while the sun is near the peak of its 11-year sunspot cycle.
During these times of “solar maximum,” dozens of sunspots, solar flares and coronal mass ejections, or CMEs, can erupt on the sun. When a solar eruption spews charged particles (mostly protons and electrons) into space, they can become entangled with the Earth’s magnetic field and generate colorful, moving lights in the upper atmosphere over the magnetic poles.
As current solar cycle 25 ramps up, aurora sightings around the world have been on the increase. Several moderate geomagnetic storms already have crossed the northern United States border, and more are in the offing.
Historically, March is the best month of the year for aurora sightings, especially around the time of the equinox on March 20. The Russell-McPherron effect suggests that the Earth’s axis at the time of the equinox is most effectively aligned with the incoming solar particles to allow them to penetrate into the Earth’s atmosphere and spark bright auroral displays.
With new sunspots popping up on the sun daily and the vernal equinox right around the corner, a new aurora season is dawning. Hopefully, the sun will start sending aurorae our way in the weeks and months ahead.
Keep an eye on the NASA website SpaceWeather.com for alerts on aurorae visible in our area.
For information about astronomy-related events in Steamboat Springs, including public star parties at CMC’s Ball Observatory, contact physics and astronomy instructor Paul McCudden, at firstname.lastname@example.org or 970-870-4537 or visit the SKY Club web page at coloradomtn.edu/skyclub.
Jimmy Westlake is adjunct Professor of Physical Sciences at Colorado Mountain College and former Director of the Rollins Planetarium at Young Harris College in Georgia and the St. Charles Parish Library Planetarium, in Luling, Louisiana. His “Celestial News” column appears monthly in the Steamboat Pilot & Today. Check out Jimmy’s astrophotography website at http://www.jwestlake.com.
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