Earth is back to rationing daylight. After summer’s surplus, we see and feel it slipping away in earlier sunsets and later sunrises. Here in Duluth, Minn. we’ve lost 40 minutes of daylight in the evening compared to the first day of summer. Sunrise times have eased back as well with a gain of another 45 minutes. Less apparent but just as significant is the length of twilight. It gets darker faster now to the tune of a half-hour at both dusk and dawn.
Like you I’ve seen this seasonal tip toward fall, but it really hit me only this morning. I got up after moonset to watch the Perseid meteor shower from a dark sky. When I arrived back home in the early light of dawn I was struck by the absence of birdsong. We get used to hearing the early birds during spring and much of the summer. This morning they were silent. Sigh.
These seasonal changes stem from a titled planet on a great yearly journey. Earth has traveled 584 million miles (940 million km) in its orbit around the sun every time you celebrate a birthday. Throughout, its axis, tipped 23.5°, remains steadfastly pointed at the North Star. On the June-July side of its orbit, the northern hemisphere faces toward the sun and the southern hemisphere away. When a subject genuflects before his king, the king appears tall and powerful. When Earth bows before its sun, it appears high in the sky. A high path brings more intense heat and long hours of daylight.
In winter, the northern half of the planet is tipped away from the sun as if recoiling from the king’s anger. The sun appears low in the sky, rising late and setting early. Of course, seasonal changes due to the Earth’s tilt are slow and leave many signs along the way. Recognizing these signs in the sky is one of the joys of being a star-watcher. Constellations change with the season, planets shift about, day-length rubber-bands, the sun climbs high, the moon drops low and on and on.
Despite missing the birds I like the shorter days. It makes it a whole lot easier to star-watch without having to wait until after 11 for a dark sky. Later sunrises also mean more time to catch up on sleep after pre-dawn forays like this morning’s. Not a single Perseid scratched my sky from 2:45 until about 3:20. Then I saw six in the next half hour including a fantastic fireball! All were swift and white. One left a train of glowing, ionized gases that persisted for about a second.
Just before dawn I could faintly see the low-slanting cone of sun-illuminated comet dusk called the zodiacal light, making August the earliest time of year I’ve noticed it. The fat, thumb-like glow of light resembling the Milky Way reached from near the horizon in Gemini up into Taurus.
Two bright vertically-aligned stars appeared in the east as the light of dawn grew — Castor and Pollux, the Gemini Twins. They’re helpful because they point the way to Mercury, which I saw dimly through haze an hour and 15 minutes before sunrise. If you’re up for Perseid viewing before dawn, tomorrow is your final day to watch the shower in a dark sky. Moonset is around 3 a.m. on the 12th, leaving an hour or so of darkness. The peak of the shower occurs Monday night when the moon will be up. Take your pick — or do both!