Highlights of the Season: July-September 2021

In 1054—and on what scholars determine to have been July 4 by modern reckoning—Chinese skywatchers took note of a “guest star” in the heavens, appearing at the tip of the southern horn of the constellation Taurus the Bull. This object gradually brightened, becoming visible in the daytime sky for several months. Even as it faded, it was still visible at night to the unaided eye for two years. This spectacular event turned out to be a supernova—the explosive death of a massive star that has about 10 times the mass of our Sun. Whatʼs left after the explosion is a tattered shell of gas known today as a supernova remnant—in this case, one which was dubbed the Crab Nebula in 1840 by an astronomer who thought thatʼs what it looked like.

On July 20, 1969, the Apollo 11 lunar module Eagle landed on the Moon, carrying Neil Armstrong and Edwin “Buzz” Aldrin to the dusty surface of the Sea of Tranquility. At the time, the Moon was almost at first quarter, so the Sun was at a low angle at the landing site. On this year's 52nd anniversary of the first human footsteps on the lunar surface, the Moon is at a waxing gibbous phase, and as seen from Tranquility Base, the Sun would be almost directly overhead. From here on Earth, look for the Moon in the south-southeastern part of the sky at sunset, about 10 degrees east of the reddish star Antares, which represents the heart of Scorpius the Scorpion.

Also, consider meteor-watching on the nights before and after the peak date, since the Eta Aquarids have more of a week-long plateau, centered on May 4. And don't forget about our informative meteor-watching video linked below!

The annual Perseid meteor shower occurs when Earth passes through the trail of dust left behind by Comet Swift-Tuttle, lasting from July 17-August 24. The dust particles burn up as they enter the atmosphere at nearly 40 miles per second and are seen as a meteor shower. When Earth is in the midst of the densest concentration of dust on August 12-13, meteor rates are highest, peaking at up to 100 per hour under ideal conditions.

This year, the peak occurs when the Moon is a waxing crescent that sets around 10 pm local time. This means that at midnight—which is the beginning of the recommended viewing window for meteor showers—there will be no interference from moonlight, and dark conditions will favor observers until the beginning of morning twilight. More background and viewing tips can be found in our “How To Observe a Meteor Shower” video, which can be found at calacademy.org/explore-science/how-to-observe-a-meteor-shower.

The full Moon in August is a blue Moon, but rather than being the second full Moon in a month (the most popular definition since 1946), the interpretation being used this time around is older and somewhat more complicated, courtesy of the Maine Farmerʼs Almanac. From 1932-57, that publication linked the term to the number of full Moons in a season. On average, a season lasting three months would have three full Moons—one per month. However, one full cycle of lunar phases is a couple of days shorter than a month, and if one of those full Moons occurs at the beginning of the season, thereʼs just enough time at the end of the season for a fourth full Moon to occur. Of those four, says the Farmerʼs Almanac, the third blue Moon in that sequence is the blue Moon. This yearʼs four full Moons between the Summer solstice on June 20 and the Autumnal equinox on September 22 occur on June 24, July 23, August 22, and September 20.

More blue moon facts:

• The expression “once in a blue Moon”—used to characterize an uncommon event—is older than either blue Moon definition discussed here and has been in use for 200 years, since 1821.
• Whichever definition you use, donʼt expect the Moon to look blue. The only time the full Moon has actually appeared that color (and it was very pale, as in the 2012 image above) is after a volcanic eruption or a forest fire has injected fine ash into the atmosphere, scattering the Moon's light.

On average, the Moon rises 50 minutes later from one night to the next, but for a few nights around the September equinox, the difference shrinks to as little as 25 minutes due to the shallow angle of the Moonʼs orbit on the horizon. This provides enough continuous light after sunset for farmers to keep harvesting crops in the fields. For this reason, the full Moon on September 20—the one closest to this yearʼs September 22 equinox—is known by tradition as the Harvest Moon. About three days after the full Moon, when the waning gibbous is already 50 percent fainter than a full Moon, the difference in moonrise times starts increasing, causing a growing gap between sunset and a gradually dimming moonrise.

Here comes the equinox again, on September 22, commonly regarded as the beginning of either fall or spring, depending on which of Earthʼs hemispheres you live on. North of the equator, it marks the start of the fall season and can also be called the fall or autumnal equinox, while to the south, itʼs the start of spring and is also known as the spring or vernal equinox. For clarity, the month is often used to reflect which hemisphere is setting the context.

More equinox facts:

• The September equinox is usually on the 22nd or 23rd of the month. On rare occasions, it has occurred on the 21st or 24th due to differences in the 365-day length of a modern calendar year and the actual 365.25-day length of Earthʼs orbital period.
• At both of Earthʼs poles, the Sun is on the horizon for 24 hours. At the equator, the Sun passes directly overhead at noon. Except at the poles, the Sun does rise and set, doing so exactly in the east and in the west, respectively, on the vernal and autumnal equinoxes. Every other day of the year, it rises and sets farther to the north or south.
• This is a good opportunity to mark the Sun's rising and setting on the horizon. If you can align these points with a foreground object—such as a gap between buildings, a street, or a tree—you can create your own “personal Stonehenge.” This has been done famously with Manhattan streets (farmersalmanac.com/manhattanhenge), and along a corridor running through the main building on the MIT campus (en.wikipedia.org/wiki/Infinite_Corridor). However, neither of these "pop-monuments" actually marks the equinoxes—can you find something better?