Bakersfield Night Sky — September 15, 2024
We’ve now finished the second weekend of “Mesmerica” at the William M Thomas Planetarium and there are just two more Saturdays to go. At the time I wrote this, there were only a few slots left in next Saturday’s (September 21) shows and the last Saturday (September 28) shows still had openings. The October shows will be on Thursdays with “Mars One Thousand One” showing on October 3 and “Black Holes” on October 24. Tickets for the October shows will open up on September 27. Mesmerica tickets are purchased through the Mesmerica ticket site while the tickets for the other regular Planetarium shows are purchased through the usual Vallitix site.
I was initially puzzled by the hype about last month’s “supermoon” that was also a “Blue Moon”. It even made the national news! I was puzzled because both of the special moons did not fit the definitions I usually use. Blue Moons come in two forms. There’s the monthly Blue Moon that is the second full moon in a calendar month with two full moons. It’s the simplest definition of a Blue Moon and it happens on average once every 2—3 years.
The other Blue Moon is a seasonal Blue Moon that is the third full moon of an astronomical season that has four full moons. An astronomical season is the time period between a solstice and an equinox or about 3 months long. The solstices mark the official start of our summer and winter seasons and the equinoxes mark the official start of our spring and autumn seasons. Our autumn equinox for 2024 is on September 22 (at 5:44 a.m. Pacific Daylight Time to be more precise, when the mid-point of the Sun is exactly on the celestial equator). Most astronomical seasons have just three full moons because the time between full moons is 29.5 days, very close to one month. Every other year or so, the timing of the full moons will make it possible to have four full moons in a season. The year 2024 was such a year and the 2024 seasonal Blue Moon occurred in the summer season. A summer Blue Moon must occur in the month of August.
The seasonal Blue Moon has a bit more complicated definition that requires one to look at four calendar months of full moons and take note of the particular dates of the equinoxes and solstices. I don’t know why the third full moon of the series is designated the seasonal Blue Moon, since every season is going to have three full moons. Why not designate the fourth full moon of the season, which for 2024 is going to be on the night of September 17/18, since it’s that fourth full moon that’s unusual? Well, no one asked me, so we’re stuck with this peculiar seasonal Blue Moon definition. Sigh! Actually, the seasonal Blue Moon definition dates back to long before my parents were born while the monthly Blue Moon definition dates back to when they were teenagers.
A seasonal Blue Moon happens about once every 2—3 years. Yes, the same frequency as the monthly Blue Moon! Now, to be fair, there are some years when two monthly Blue Moons occur within one year but sometimes a seasonal Blue Moon can happen less than two years after the previous seasonal Blue Moon. Those closer spacing Blue Moons happen roughly every 20 years. You know, once every, uh… purple moon. Since the monthly Blue Moon requires just looking at the full moons in just one calendar month, I prefer the monthly Blue Moon definition. The next monthly Blue Moon is going to be in May 2026 and the next seasonal Blue Moon will be in May 2027.
Now for the “supermoon”. Last month’s supermoon fit the definition used by NASA’s human moon calculator, Fred Espenak (now retired). NASA’s (Espenak’s) definition says that if the full moon’s “relative distance” is at least 0.9, it is a supermoon. The “relative distance” is defined as (distance of the moon at apogee - distance of the full moon) / (distance of the moon at apogee - distance of the moon at perigee). If the full moon’s distance is at perigee, then the relative distance = 1. With this definition of a supermoon, most years will have at least 3 supermoons and some years can have up to five supermoons. Also, one needs an Espenak calculator because there is some slight variation of the apogee and perigee distances every month (flexure of the moon’s orbit) due to the sun’s gravitational influence on the moon’s eccentric orbit.
The other definition of supermoon I prefer to use is a full moon happening within 24 hours of perigee. Besides being something you can write down on one line, this supermoon is a subset of the NASA supermoon, so it’s truly more extraordinary. This month’s full moon is a supermoon of the simpler definition and we’ll also be able to see a partial lunar eclipse when the moon passes through the bottom of Earth’s penumbra shadow and the bottom tip of Earth’s umbra just barely touches the top of the moon (see the star chart above). Because of this, it will be challenging to detect any change in the full moon’s brightness but look for the umbra touching the full moon between 7:12 and 8:17 p.m. (Pacific Time) on September 17. Look for it low in the east below the Great Square of Pegasus.
September’s supermoon is almost 2800 miles closer than August’s full moon. Next month’s full moon will be the closest supermoon of the year, 76 miles closer than this month’s supermoon. The closest supermoon of the twenty-first century will be on December 5, 2052 (Pacific Time) when the full moon will be just 221,475 miles away or 581 miles closer than October’s supermoon.
—
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com
