September 19, 2023: Venus has a strangely-named geometrical configuration in the solar system today. Find four bright planets overnight and the crescent moon after sunset.
by Jeffrey L. Hunt
Chicago, Illinois: Sunrise, 6:35 a.m. CDT; Sunset, 6:53 p.m. CDT. Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.
Venus is at a one-of-a-kind geometric configuration with Earth and Sun today. Technically, it is called the greatest illuminated extent. This occurs when the planet is brightest in our sky. The configuration occurs before inferior conjunction when Venus is an Evening Star and now in the morning sky.
Through a telescope, Venus shows a morning crescent phase 27% illuminated. The angle from the sun to Venus, with Earth at the vertex, is 40°.
The Venusian crescent covers the largest area of the sky. This seems counterintuitive when we think about the moon. When the moon is full the illuminated globe covers the largest area of the sky compared to the illumination of a lunar crescent. This occurs because the moon is about the same distance from Earth throughout its lunar cycle.
Venus’ distance varies along with its phase. When Venus passed between Earth on August 13th, it was nearly 27 million miles from our planet. The Earth-facing side was (for discussion purposes here) not illuminated, like a New moon. This morning, it is over 40 million miles distant. The distance increases as Venus moves away from us and its phase grows. The morning half phase occurs October 23rd, when the planet is 65 million miles away, but the planet appears 35% smaller through a telescope. While the phase grows, it appears smaller, covering less of the sky than this morning.
By December 5th, Venus is 93 million miles from Earth with a morning gibbous phase that is 70% illuminated, but it is 56% smaller than this morning’s telescopic size. The planet is farther away, smaller in appearance, and dimmer in the sky, although it is still the brightest starlike body in the sky.
As the planet passes behind the sun at superior conjunction late next spring, its phase grows to full, but it is 75% smaller than today’s size.
So today, while the phase is a crescent, it covers the largest area of the sky – known as the greatest illuminated extent. For those readers who want more information about this effect, see this source. Step outside this morning to see a brilliant Venus. How ever this occurs, Venus is dazzling in the morning sky.
Summaries of Current Sky Events
Summary for Venus as a Morning Star, 2023-24
Here is today’s planet forecast:
Morning Sky
Three bright planets are visible along the arc of the ecliptic at two and one-half hours before sunrise. Saturn is the most difficult to see at this hour, about 5° above the horizon in the west-southwest, to the lower right of Skat and Lambda Aquarii (λ Aqr on the chart).
Bright Jupiter, the second brightest starlike body in the sky this morning, is high in the southern sky. It is retrograding in front of Aries, 13.4° to the lower left of Hamal, the Ram’s brightest star, 11.2° to the upper right of Menkar, in Cetus, and 16.0° to the lower right of the Pleiades star cluster, part of Taurus.
At this hour, brilliant Venus is over 7° above the eastern horizon. This view of the three planets at the same time is becoming a challenging observation, mainly from Saturn’s brightness. It is lower each morning and starting to shine through the lower layers of the air near the horizon that blurs and reddens celestial objects. This is most obvious with a rising or setting sun and moon. Venus and Jupiter are bright enough to be seen near an unobstructed horizon, but not Saturn.
Venus and Saturn are 180° apart on October 10th. After this date, Saturn sets before Venus rises, but with the atmospheric filtering of Saturn’s light, Saturn may disappear before Venus rises during the next several days. One way to lengthen this observation in the short term is to begin looking for them about three hours before daybreak.
By one hour before sunup, Venus, appearing as a bright Jewel of the morning, is over 25° up in the east. It is the same altitude – height above the horizon – as Sirius, the Dog Star, that is over 40° to the planet’s right in the southeastern sky.
The Little Dog Star, Procyon, is above an imaginary line from Venus to Sirius.
Venus is stepping eastward toward Regulus, 14.7° to the planet’s lower left. Venus passes the star October 9th.
Mercury is putting on its best morning display of the year, about 5° above the horizon and 8.9° to the lower left of Regulus at this hour.
Wait another fifteen minutes to see Mercury higher, over 7° above the horizon. The planet reaches its largest separation from the sun in three mornings. It brightens each morning. Use a binocular to initially find the speedy planet.
Evening Sky
After the challenging-to-see grouping of Mars and the moon a few days ago, the Red Planet continues to slip into bright sunlight. This evening it sets less than forty-five minutes after nightfall.
At this time, the crescent moon, 22% illuminated, is about 10° above the southwest horizon, 7.9° to the lower left of Zubenelgenubi, the Scorpion’s southern claw, and 10.5° to the lower right of Dschubba, the critter’s forehead.
Scorpius’ brightest star, Antares, is 7.6° to the left of Dschubba and less than 20° to the upper left of the lunar orb.
Look for earthshine between the cusps or horns of the moon. This is reflected sunlight from Earth’s clouds, oceans, and land. Photograph it with a tripod-mounted camera and exposures up to a few seconds, depending on the camera’s settings.
Saturn, not as bright as Venus or Jupiter, is over 25° above the southeast horizon at two hours after sunset. The planet rises in the southeast before sunrise, appearing higher during the early evening hours.
Saturn appears to be moving westward compared to the starry background of the constellation Aquarius. This retrograde motion is from Earth passing between the planet and the sun. The Ringed Wonder is 9.5° to the upper right of Skat and 9.9° to the right of Lambda Aquarii.
Look for the star Fomalhaut, about 20° below Saturn and about 10° above the horizon. The name means “the mouth of the southern fish.”
At three hours after sundown, bright Jupiter is over 10° up in the east.
During the night, as Earth rotates, the stars and planets seem to shift westward in unison. Before twilight begins tomorrow, Venus, Jupiter, and Saturn are visible at the same time.
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