The Celestial Sphere

By Matthew Moore Johnson

On Sept. 23, at 2:50 a.m., the Earth will experience the autumnal equinox. The term equinox derives from the Old French equinoxe or Latin aequinoctium (from aqui, equal + nox, noct, night).

The equinox is a result of the Earth’s being tilted on its axis 23 1/2 degrees. Thus, during Earth’s orbit the sun illuminates either the Northern or the Southern Hemisphere a bit more than the other depending on the planet’s location in its orbit. However, during an equinox—of which there are two, the vernal equinox in March and the autumnal equinox in September—the sun shines directly over the equator as the Earth rotates. Therefore, both the Northern and Southern Hemispheres receive almost exactly the same amount of sunlight and darkness, close to 12 hours of each. After the autumnal equinox, the sun starts heading southward, and our days in the Northern Hemisphere begin to lose sunlight as each day passes. On Sept. 1 we experience 13 hours and 10 minutes of daylight; by the month’s end we have lost 1 hour and 22 minutes of sunlight, with Sept. 30 having only 11 hours and 42 minutes of daylight.

There are three basic ways of scientifically defining autumn:

1. Astronomically, the seasons are considered to center around the equinoxes and the solstices—points in the Earth’s orbit about the sun. There is disagreement between those who see the equinoxes and solstices as the beginning of the seasons and those who regard the equinox or solstice as the middle of the season.

2. Meteorologists generally divide the seasons into periods of three whole months, based on average monthly temperatures. Employing this criterion, meteorologists in the Northern Hemisphere consider September, October and November to be the months of autumn. 

3. Ecologists and botanists use “phenological indicators” to define autumn, for example, when certain plants first appear, when leaves start to fall, and when different birds and other animals begin migrations. The dating of these indicators are in flux as the climate is changing, which is beginning to push or pull the dates of the onset of autumn and the other seasons. 

Meteor showers:

There are no meteor showers of note in the month of September. 

Planets: 

Mercury: The best morning view of Mercury is on the 20th when it rises 90 minutes before sunrise. On the 29th, Mercury rises to 9 degrees above the horizon 30 minutes before sunrise while the planet Venus looms above it.

Venus: Venus is found shining brightly in the morning in the constellation Cancer an hour before the sun rises. On the 9th, Venus reaches its greatest brilliancy. At this time, Venus is bright enough to create shadows. Since Venus orbits between the Earth and the sun, it goes through phases, just as our moon does. On the 25th, Venus enters the constellation Leo and can be seen just west of Leo’s brightest star, Regulus. 

Mars: Mars’s position in relation to the sun makes it difficult to view in September.

Jupiter: Jupiter rises about 10 p.m. during the first week of September. It is easy to spot, glowing brilliantly against the background of stars. A standard set of binoculars reveals the planets four Galilean moons: Io, Europa, Ganymede and Callisto. Even a small telescope can sometimes bring the two main storm bands of Jupiter into view. In a larger telescope, shadows of the four moons when they pass in front of Jupiter can be seen traveling across the top of Jupiter’s atmosphere.

The James Web Space Telescope was designed to view extremely distant objects created very early in our cosmos by capturing red-sifted light coming from them. Since its launching on Dec. 25, 2021, the telescope has performed perfectly, imaging galaxies created less than 500 million years after the Big Bang. Scientists were shocked to learn that these early galaxies were massive and were emitting extraordinary amounts of light. Since this information was released in July 2022, astronomers and cosmologists have had to reconsider much of their understanding of the evolution of our cosmos. 

Moon Phases:

Last quarter: Sept. 6, left half of moon illuminated

New moon: Sept. 14, no moon visible

First quarter: Sept. 22, right half of moon illuminated

Full moon: Sept. 29, entire face of moon illuminated, the Harvest Moon

The moon produces no light of its own. It reflects the light being emitted by other objects. For the most part, the sun’s rays illuminate the phases of the moon, and the moon reflects the sun’s rays to the Earth. The stars also illuminate the moon somewhat, and when the Earth is positioned appropriately, the sun’s rays striking the oceans and clouds reflect the sun’s light back onto the moon’s surface with a faint illumination termed “earthshine.”

September’s full moon is termed the Harvest Moon. The Harvest Moon occurs near the autumnal equinox and rises around sunset for several nights. The moon rising at sunset provided farmers with additional light to continue with the harvest in the evening. September’s full moon is also termed the Corn Moon.