Highlights of the Season: July-September 2023

In July and part of August, Mars is visible low in the west just after sunset, and yes, it has seasons—summer on the Red Planet starts on July 12. Like Earth, Mars has an axial tilt and it maintains a constant orientation with respect to the stars, so each Martian year, as the Red Planet orbits the Sun, it alternates between leaning its north pole toward, then away from the Sun. The difference is that Mars is half-again as far from the Sun than we are, so its orbit takes longer—roughly twice as long as Earth's, or 687 days. Hence, its seasons are correspondingly about twice as long as Earth's.

During summer in the northern hemisphere, the north polar ice cap shrinks to its smallest size, and observers on Earth will be hard-pressed to see it in telescopes. While the average temperature on the surface is about -80°F (-62°C), summertime temperatures on the equator can reach a balmy 76°F (71°C).

Have you ever wondered where the expression "dog days of summer" came from? In ancient times, the Greeks realized that during the year, the Sun slowly moved eastward against the constellations, its bright glow occasionally blocking some stars from view as it came into conjunction with them. Those stars weren't seen again until the sun moved far enough eastward for the stars to emerge from the glare and become briefly visible in the morning sky.

One of those stars was Sirius in the constellation Canis Major the Big Dog. Also known as the brightest star in the entire night sky, Sirius reliably became visible just before dawn when the Sun moved far enough eastward so it could be seen rising before it was almost immediately washed from view by the growing glow of morning twilight. In the days following, it would rise earlier and be visible for a few minutes longer as its angular distance from the Sun increased. That first, brief appearance, however, is called the star's heliacal rising, and for the ancients, it occurred reliably in mid-July (nowadays, due to changes caused by a slow wobble of Earth's axis, it happens later, around August 8-1.) Since Sirius is in the constellation of the Big Dog and was sometimes referred to as the Dog Star, the hot, summer days that followed its heliacal rising became known as the Dog Days.

The Delta Aquarid meteor shower is active from July 18-August 21, with a diffuse peak of around 20 faint meteors per hour in late-July, under ideal conditions. According to the American Meteor Society (www.amsmeteors.org/meteor-showers/meteor-faq/), this rate is reduced to about 3-5 per hour in suburban areas and in the interfering light of a waxing gibbous Moon. With a radiant point of the shower positioned in the southerly constellation Aquarius the Water-Carrier, this display favors observers south of the Equator, where the radiant is higher in the sky.

The Delta Aquarids peak about two weeks before the peak of the Perseid meteor shower, and the fringes of the two showers actually overlap each other. The Perseids benefit from abundant meteors, warm and usually clear summer nights, and an obligingly non-intrusive Moon phase. Known from Chinese records dating to 36 AD, this display is active starting in mid-July, when Earth enters the trail of dust particles left behind by Comet Swift-Tuttle. At first, the number of meteors is no different from the 4-6 sporadic meteors seen on any clear night, but the rate slowly increases, building to a peak that occurs during the morning hours of August 12th (although experienced observers know to watch for several nights before and after the peak). That's when the shower may produce 50-75 bright, swift meteors per hour under perfect conditions, according to the International Meteor Organization (https://imo.net/resources/calendar). Then, numbers quickly decline by September 1, when Earth exits the dust trail.

Here's our video about how to observe meteor showers: https://www.youtube.com/watch?v=EBF4wFhw2Kg

The full Moon on August 31 is popularly called a blue Moon because it's the second full Moon in a single calendar month, which happens about seven times every 19 years. In this usage, a full Moon would occur during the first few days of the month and again in the last few days, because 11 months of the year are just long enough to encompass a complete 29½-day cycle of lunar phases (the exception, of course, being February). However, this usage is the result of a misinterpretation made in 1946 of an older, more traditional definition from the 1937 Old Farmer's Almanac. There, a blue Moon was described as the third occurrence during a season that has four full Moons—something that happens about every 2½ years. The next such seasonal blue Moon occurs on August 19, 2024.

The expression "once in a blue Moon" is commonly used to refer to a rare occurrence, to which either definition can apply—but whichever you prefer, the Moon doesn't actually change color and turn anything near a shade of blue.

The autumnal (or fall) equinox for the northern hemisphere occurs on September 22 at 11:50 pm PT.

• Because the Sun's path against the constellations is tilted from the plane of Earth's equator, it spends half the year in the northern celestial hemisphere and the other half in the southern celestial hemisphere. The equinoxes are when it crosses the celestial equator, and in the case of the September equinox, it's passing into the southern half of the celestial sphere.
• Lying on the celestial equator (the extension of Earth's equatorial plane into space), the Sun rises exactly in the east and sets exactly in the west because those points are where the celestial equator intersects the horizon.
• A common belief is that on the year's two equinoxes (fall and spring, occurring in September and March), day and night are of equal length, but it's not that simple. Most sunrise and sunset tables are calculated according to where the center of the Sun's disk is with respect to the horizon rather than its edge, effectively reducing the Sun to a single point. In reality, the solar disk has a diameter of ½° of arc, and as the Sun rises, the top edge of the disk crosses the horizon about a minute before the center does. Likewise, upon setting, the center of the disk crosses the horizon a minute or so before the top edge does. In addition, refraction, or the bending of light by the atmosphere, creates a mirage of the Sun that appears slightly earlier than actual sunrise and which is likewise visible after actual sunset. This means that some part of the Sun's disk is visible for several minutes longer than is calculated for the length of a given day. The days that the Sun is above and below the horizon for equal amounts of time vary with latitude, but are a few days before the spring equinox and a few days after the autumnal equinox. For latitude 40° North, those dates—called equilux—are March 17 and September 26.
• In the southern hemisphere, the September equinox is the first day of Spring.

The full Moon on September 29 is the closest full Moon to the Autumnal equinox and so is traditionally known as the Harvest Moon. This name refers to the fact that for a few nights, the Moon seems to rise at much less than the usual 50-or-so-minute interval from one night to the next. In the days before electric lights, this afforded farmers working in the field more continuous natural illumination under which to harvest crops after sunset, without a gap of darkness between the end of useful twilight and moonrise.

According to the Islamic calendar, September's new Moon on the 14th marks the start of the month Rabi' al-Awwal which means "first month of Spring." However, this name comes from pre-Islamic times (before 622 AD), when a lunisolar calendar was used in which seasons and months were more synchronized. The Islamic calendar now in use is based purely on the Moon.

In the lunar calendar, each month is 29½ days long, as measured by a complete cycle of lunar phases, from one new Moon to the next. This means that the Islamic year of 12 lunar cycles is 254 days long, unlike the Western 365-day year, which is based on the apparent motion of the Sun. Because of this, the dates of major observances in the lunar calendar occur 11 days earlier from year to year in the solar calendar, and over enough time they migrate through the seasons, taking about 33 years to resynchronize. The last time the first day of Rabi' al-Awwal fell close to the Vernal equinox was in 2007. The two systems will match up with each other again (more or less) in 2039.