June 21 marks our summer solstice, the longest day in the northern hemisphere. It is conversely the winter solstice, the shortest day, in the southern hemisphere. (Yes, there’s an entire, pesky other hemisphere down there, where everything is the other way around.)

Up north here, we call June 21 the first day of astronomical summer. That word, “astronomical,” distinguishes June 21 from the climatological (a ten-dollar word for “hottest”) weeks of summer, running mid-July to late August.

But wait a minute… if the longest day is June 21, why is that not the warmest day, with the peak temperature of the season? The reason is that the atmosphere has a lot of mass, with a lot of thermal inertia that takes time to shift before attaining maximum seasonal temperatures.

It’s like pushing on a mining railcar with a monocled rhinoceros in it, to start the car rolling down the tracks. It takes a long time and a lot of effort to get it moving. (Why a rhinoceros with a monocle is sitting in a mining car, we cannot say. But now that you’ve got that image for inertia, you’ll never forget it.)

We might wonder, though, what exactly makes summer hot? We posed this question to Luna, our resident astronomer. She emailed back to us with this column’s four-part June puzzle.

She comments, “It’s easy to take seasonal changes for granted, with perhaps only a vague appreciation for what makes them happen. Here are four possibilities for what brings on our Nederland summer.”

“The first might be the sun’s high summer angle above the horizon. This angle maximizes at 73.5 degrees above the horizon at about noon (1 p.m. daylight savings time) in Nederland on June 21. Maybe large sun angles make for large vertical components (available for absorption) of solar rays hitting the atmosphere and the ground. But is that right?”

She goes on, “Another possible cause of summer is longer day length, equating to more daily hours of heating. But again, does this add up?”

Luna cites a third possibility, “The earth’s orbit is elliptical - the sun’s distance varies through the year. Does our summer in Nederland come on because we are closer to the sun in summer than in winter?”

Lastly, she suggests that Volcano Gnomes, who live in Iceland (really—look it up) and Sicily and Hawaii and Reunion Island, might be responsible for summer heating.

Luna’s challenge to our Faithful Readers, which she’ll answer next month, is: Which of these four possibilities causes our Nederland summers? (Hint: There may be more than one correct answer.)

Backyard Astronomy Lecture Series: This correspondent is conducting a lecture series, Backyard Astronomy, at the Nederland Public Library on the first Wednesdays of May through August, with a bonus talk, How Far is That Star, on the first Wednesday of September. Details and registration at https:// nederland.colibraries.org/backyardastronomy/. The next talk, at 6 p.m. on June 7, will cover essentials of telescopes and binoculars.

Also in June skies: The sun begins the month in Taurus, entering Gemini on June 21. On the solstice, days and nights are 15 and 9 hours long, respectively.

The moon’s dates are: Full (Strawberry Moon) June 3; Last Quarter June 10; New June 18; First Quarter June 26.

June Meteor Showers: The Arietids (parent, either asteroid 1566 Icarus or comet 96P/Machholz) and Bootids (ba-OO-tids) (parent, comet 7P/Pons-Winnecke) peak on June 11 and 27 respectively. The Arietids are visible in Nederland just before, and even a bit after, sunrise; the Bootids are especially good just after dusk.

Best Sky Viewing Nights (Minimal Moon): June 10-26.

Sunset (Mid-Month): Venus and Mars are low in the west. Leo with Regulus is prominent above them and to their left, in the southwest; the Gemini Twins Castor and Pollux are below them and to their right. Arc from the Big Dipper’s arm to red Arcturus, then on to bright-white Spica in the south.

Midnight (Mid-Month): Vega, Deneb and Altair form the Navigator’s Triangle, spanning the northeast to nearly overhead.

Sunrise (Mid-Month): The Great Square of Pegasus is just eastward of being overhead. The Pleiades rise just ahead of the sun, while Hercules is low in the west.

Mercury rises just ahead of the sun at the beginning of the month; it is at its highest altitude on the morning of June 4. After that it sinks back into the sun.

Venus is low in the evening sky. Look for it next to the thin crescent moon after sunset on the solstice.

Mars is faint in the evening sky near Venus, staying between Regulus (above Mars and to its left) and Castor and Pollux (below Mars and to its right).

Jupiter is very low in the east at sunrise.

Saturn rises before the sun, low in the southeast at dawn.

Notable Space Missions: The joint European Space Agency and Japan Aerospace Exploration Agency (ESA-JAXA) Bepi-Colombo probe will make its third Mercury flyby on June 20. It will begin orbiting Mercury, with two sub-satellites, in December 2025.

Frank Sanders, a spectrum scientist at the U.S. Department of Commerce in Boulder, takes astronomy-related inquiries at backyardastronomy1@gmail.com.