The September full moon, known as the “Corn Moon”, always shines in or near the stars of Aquarius and Pisces. Because the moon appears full only when opposite the sun, full moons always rise around sunset and set around sunrise.
The Season is Changing
There’s a lot of unusual stuff going on around tonight’s full moon. First, it will rise to appear situated in the constellation of Aquarius. Second, and it will find its trajectory passing near Neptune as the blue planet will have been sitting in opposition on Tuesday evening; the point at which the planet will have appeared brightest in our sky.
To see the full moon and Neptune at the same time, you’ll need to have a telescope, or at least binoculars, because far-away Neptune isn’t visible to the naked eye. According to Space.com, the moon and Neptune will pass within 0.73 degrees of each other around 1 a.m. Eastern on Wednesday, with Neptune hanging out just to the northwest of the moon.
The moon will be officially full at 3 a.m. Eastern Wednesday, so plan to stay up late tonight.
Traditionally, September’s full moon is always called the Harvest Moon, but not this time around. Since the Harvest Moon is always the full moon that arrives closest to the fall equinox (September 22), the full moon on October 5 will get the iconic title. September’s full moon, instead, has been deemed the Corn Moon. It’s been the go-to name for the full moon when it falls far enough away from the fall equinox, as First Nations tribes always gave the moon names that reflected aspects of the season.
As the Corn Moon, or alternately the Barley Moon, early September’s moon name reflects the crops that were ready for harvest around this time. It’s also known as the Falling Leaves Moon and the Nut Moon.
The equinoxes occur twice a year when the tilt of the Earth’s axis is inclined neither away from nor towards the Sun, the Sun being vertically above a point on the Equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name “equinox” is derived from the Latin aqueous (equal) and Nox (night), because around the equinox, the night and day are approximately equally long. At an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points: the vernal point and the autumnal point. An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location on the Earth’s Equator where the centre of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year.
- Autumnal equinox: this classical name is a direct derivative of Latin (autumnus = autumn).
- September equinox: a usage becoming the preferred standard by technical writers choosing to avoid Northern Hemisphere bias (implied by assuming that March is in the springtime and September is autumnal—true for those in the Northern Hemisphere but exactly opposite in the Southern Hemisphere).
- Northward equinox and southward equinox: names referring to the apparent motion of the Sun at the times of the equinox.
- Vernal point and autumnal point are the points on the celestial sphere where the Sun is located on the vernal equinox and autumnal equinox respectively (again, the seasonal attribution is that of the Northern Hemisphere).
- First point (or cusp) of Aries and first point of Libra are names used by navigators and astrologers. Navigational ephemeris tables record the geographic position of the First Point of Aries as the reference for the position of navigational stars. Due to the precession of the equinoxes, the astrological signs where these equinoxes are located no longer correspond with the actual constellations once ascribed to them.
Length of Equinoctial Day and Night
On a day of the equinox, the centre of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, night and day being of roughly the same length. The word equinox derives from the Latin words aqueous (equal) and Nox (night); in reality, the day is longer than the night at an equinox. Commonly, the day is defined as the period when sunlight reaches the ground in the absence of local obstacles. From the Earth, the Sun appears as a disc rather than a single point of light, so when the centre of the Sun is below the horizon, its upper edge is visible. Furthermore, the atmosphere refracts light, so even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. These cumulative effects make the day about 14 minutes longer than the night at the Equator and longer still towards the Poles. The real equality of day and night only happens in places far enough from the equator to have a seasonal difference in day length of at least 7 minutes, actually occurring a few days towards the winter side of each equinox. The date at which the time between sunset and sunrise crosses 12 hours, is known as the equinox. Because sunset and sunrise times vary with an observer’s geographic location (longitude and latitude), the equinox likewise depends on location and does not exist for locations sufficiently close to the equator. The equinox, however, is a precise moment in time which is common to all observers on Earth.
Heliocentric view of the seasons
The Earth’s seasons are caused by the rotation axis of the Earth not being perpendicular to its orbital plane. The Earth’s axis is tilted at an angle of approximately 23.44° from the orbital plane; this tilt is called the axial tilt. As a consequence, for half of the year (i.e. from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum around June 21, while for the other half of the year, the southern hemisphere has this honour, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North and South Poles of the Earth are just on the terminator and day and night are divided equally between the hemispheres. A few remarks can be made about the equinoxes:
- Because the Sun is a spherical (rather than a single-point) source of light, the actual crossing of the Sun over the Equator takes approximately 33 hours.
- At the equinoxes, the rate of change for the length of daylight and night-time is the greatest. At the Poles, the equinox marks the start of the transition from 24 hours of nighttime to 24 hours of daylight. High in the Arctic Circle, Longyearbyen, Svalbard, Norway has an additional 15 minutes more daylight every day around the time of the Spring equinox, whereas in Singapore (which is virtually on the Equator), the amount of daylight each day varies by just seconds.
- It is 94 days from the June solstice to the September equinox, but only 89 days from the December Solstice to the March equinox. The seasons are not of equal length, because of the variable speed of the Earth in its orbit around the Sun.
- The instances of the equinoxes are not fixed, but fall about six hours later every year, amounting to one full day in four years. They are reset by the occurrence of a leap year. The Gregorian calendar is designed to follow the seasons as accurately as is practical, which is good, but not absolutely perfect.
- Smaller irregularities in the times are caused by perturbations of the Moon and the other planets.
- Currently, the most common equinox and solstice dates are March 20, June 21, September 22 and December 21; the four-year average will slowly shift to earlier times in coming years. This shift is a full day in about 70 years (compensated mainly by the century “leap year” rules of the Gregorian calendar). This also means that in many years of the twentieth century, the dates of March 21, June 22, September 23 and December 22 were much more common, so older books teach (and older people may still remember) these dates.
- Note that the times are given in UTC (roughly speaking, the time at Greenwich, ignoring British Summer Time). People living farther to the east (Asia and Australia), whose local times are in advance, will see the seasons apparently start later; for example, in Tonga (UTC+13), an equinox occurred on September 24, 1999, a date which will not crop up again until 2103. On the other hand, people living far to the west (America) whose clocks run behind UTC may experience an equinox as early as March 19.
Geocentric view of the seasons
In the half year centred on the June solstice, the Sun rises and sets towards the north, which means longer days with shorter nights for the Northern Hemisphere and shorter days with longer nights for the Southern Hemisphere. In the half year centred on the December solstice, the Sun rises and sets towards the south and the durations of day and night are reversed. Also on the day of an equinox, the Sun rises everywhere on Earth (except the Poles) at 06:00 in the morning and sets at 18:00 in the evening (local time). These times are not exact for several reasons, one being that the Sun is much larger in diameter than the Earth that more than half of the Earth could be in sunlight at any one time (due to unparallel rays creating tangent points beyond an equal-day-night line); other reasons are as follows:
- Most places on Earth use a time zone which is unequal to the local time, differing by up to an hour or even two hours, if daylight saving time (summer time) is included. In that case, the Sun could rise at 08:00 and set at 20:00, but there would still be 12 hours of daylight.
- Even those people fortunate enough to have their time zone equal to the local time will not see sunrise and sunset at 06:00 and 18:00 respectively. This is due to the variable speed of the Earth in its orbit and is described as the equation of time. It has different values for the March and September equinoxes (+8 and −8 minutes respectively).
- Sunrise and sunset are commonly defined for the upper limb of the solar disk, rather than its centre. The upper limb is already up for at least one minute before the centre appears, and likewise, the upper limb sets one minute later than the centre of the solar disk. Due to atmospheric refraction, the Sun, when near the horizon, appears a little more than its own diameter above the position than where it is in reality. This makes sunrise more than another two minutes earlier and sunset the equal amount later. These two effects add up to almost seven minutes, making the equinox day 12hrs 7min long and the night only 11hrs 53min. In addition to that, the night includes twilight. When dawn and dusk are added to the daytime instead, the day would be almost 13 hours.
- The above numbers are only true for the tropics. For moderate latitudes, this discrepancy increases (for example, 12 minutes in London) and closer to the Poles it gets very large. Up to about 100 km from both Poles, the Sun is up for a full 24 hours on an equinox day.
- The height of the horizon on both the sunrise and sunset sides changes the day’s length. Going up into the mountains will lengthen the day while standing in a valley with hilltops on the east and the west can shorten the day significantly. This is why settlements in east-west running valleys are more favourable (daylight-wise) than north-south running valleys.
Cultural Aspects of the Equinox
- The September equinox marks the first day of Mehr or Libra in the Persian calendar. It is one of the Iranian festivals called Jashne Mihragan, or the festival of sharing or love in Zoroastrianism.
- The Mid-Autumn Festival is celebrated on the 15th day of the 8th lunar month, often times near the autumnal equinox day, and is an official holiday in China and in many countries with a significant Chinese minority. As the lunar calendar is not synchronous with the Gregorian calendar, this date could be anywhere from mid-September to early October.
- The traditional harvest festival in the United Kingdom was celebrated on the Sunday of the full moon closest to the September equinox
- The September equinox was “New Year’s Day” in the French Republican Calendar, which was in use from 1793 to 1805. The French First Republic was proclaimed and the French monarchy was abolished on September 21, 1792, making the following day (the equinox day that year) the first day of the “Republican Era” in France. The start of every year was to be determined by astronomical calculation, (that is: following the real Sun and not the mean Sun as all other calendars).
- World Storytelling Day is a global celebration of the art of oral storytelling, celebrated every year on the spring equinox in the northern hemisphere, the first day of autumn equinox in the southern.
- Wiccans and many other Neopagans hold religious celebrations of “Mabon” on the autumnal equinox.
Myths, Fables and Facts
- One effect of equinoctial periods is the temporary disruption of communications satellites. For all geostationary satellites, there are a few days near the equinox when the sun goes directly behind the satellite relative to Earth (i.e. within the beamwidth of the ground-station antenna) for a short period each day. The Sun’s immense power and broad radiation spectrum overload the Earth station’s reception circuits with noise and, depending on antenna size and other factors, temporarily disrupt or degrade the circuit. The duration of those effects varies but can range from a few minutes to an hour. (For a given frequency band, a larger antenna has a narrower beamwidth, hence experiences shorter duration “Sun outage” windows).
- Although the word equinox is often understood to mean “equal [day and] night,” as is noted elsewhere, this is not strictly true. For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal; those days are commonly referred to as the “equinoxes” to distinguish them from the equinoxes. Equinoxes are points in time, but equinoxes are days. By convention, esquilaxes are the days where sunrise and sunset are closest to being exactly 12 hours apart. This way, you can refer to a single date as being the Equiluz, when in reality, it spans from sunset on one day to sunset the next, or sunrise on one to sunrise the next.
- What is true about the equinoxes is that two observers at the same distance north and south of the equator will experience nights of equal length.
- The Equiluz counts times when some direct sunlight could be visible, rather than all hours of usable daylight (which is any time when there is enough natural light to do outdoor activities without needing artificial light). This is due to twilight; a particular type of twilight which is officially defined as civil twilight. This amount of twilight can result in more than 12 hours of usable daylight up to a few weeks before the spring equinox, and up to a few weeks after the fall equinox.
- In a contrary vein, the daylight which is useful for illuminating houses and buildings during the daytime and is needed to produce the full psychological benefit of daylight is shorter than the nominal time between sunrise and sunset. So in that sense, “useful” daylight is present for 12 hours only after the vernal equinox and before the autumnal equinox, because of the intensity of light near sunrise and sunset, even with the sun slightly above the horizon, is considerably less than when the sun is high in the sky.
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