September 1, 2024: Mercury appears in the morning sky during early September. It joins the nightly parade of five bright planets through mid-month.
by Jeffrey L. Hunt
Chicago, Illinois: Sunrise, 6:17 a.m. CDT; Sunset, 7:23 p.m. CDT. Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.
Nightly Planet Parade
Five bright planets are visible during the nighttime. From sunset to sunrise, the five planets march westward. Begin by looking for brilliant Venus 30 minutes after sunset, low in the western sky. Unlike most celestial bodies, Venus shines through bright twilight. It continues to migrate southward along the horizon during the month.
Saturn
Tonight, Saturn rises only 18 minutes after the sun sets, but it is too dim to see with Venus as the Evening star sets about 40 minutes later. They first become visible simultaneously during evening twilight on about the 10th.
Jupiter and Mars Appear Near Midnight
As Earth rotates during the night, Saturn is higher in the east-southeast, crossing the south point around midnight, when bright Jupiter rises followed about 30 minutes later by Mars. The Jovian Giant and Red Planet are with Taurus to the lower left of Aldebaran, Taurus’ brightest star.
During the night the planet triplet shifts farther westward. During morning twilight, Saturn is low in the west-southwest, while Jupiter and Mars are high in the east-southeast.
Mercury is Parade’s Street Sweeper
The fifth planet in the parade rises 85 minutes before the sun in the east-northeast. This morning it is near the razor-thin moon.
The nightly bright planet parade continues until about mid-month when Mercury plunges back in to morning twilight.
Mercury’s September Morning Appearance
Here is the forecast for Mercury’s September appearance:
Mercury loops from the eastern predawn sky to a post sunset view and back to the morning sky about every four months.
Since the speedy planet is closer to the sun than Earth, Mercury always appears near the central star and rarely in a completely dark sky. Mainly, it is visible during mid-twilight.
Because Mercury’s orbit is eccentric, its largest separation from the sun is less than 30°. When near its closest point to the sun, known as perihelion, the separation can be as little as 18°. Mercury’s perihelion occurs on the 9th.
Because Earth is tilted, the angle the ecliptic, the solar system’s plane, makes with the horizon is very shallow or steep. At this season the angle is very high, near its maximum value. Even with a small separation from the sun, the planet is visible from a clear horizon looking toward the planet.
After its inferior conjunction, between Earth and sun on August 18th, it raced toward the sun’s west, rising up to eight minutes earlier each morning.
This morning at 45 minutes before sunrise, Mercury is nearly 7° above the east-northeast horizon. It is nearly as bright as Procyon, the Little Dog Star, nearly 30° to the planet’s upper right, although Mercury’s brightness is dimmed and blurred by the thicker air near the horizon.
Mercury is 4.4° to the lower right of razor-thin crescent moon, that is only 2% illuminated. The pair easily fits into the same binocular field and might be the only manner to see the lunar crescent.
Greatest Elongation
Each morning during the early days of September, Mercury rises two to three minutes earlier and brightens. It reaches its greatest separation, 18°, known as the greatest elongation or largest angle from the sun, on the 4th.
At its greatest elongation, the Sun-Earth-Mercury angle is at its largest. If the planet’s orbit were a circle, the Earth-Mercury side would be tangent to a line from Mercury to the sun.
If the orbit could be visualized, it would have a high angle to the eastern horizon, with Mercury at the top, 18° above the sun at daybreak.
At 45 minutes before daybreak, Mercury – brighter than all the stars in the eastern sky except for Sirius and Jupiter, is nearly 8° above the east-northeast horizon. This is the best we see it during this appearance – a bright planet shining low in the east-northeast during morning twilight.
The planet is nearly 6° to the upper right of Regulus, Leo’s brightest star, emerging from bright morning twilight after its solar conjunction, but not yet visible to the unassisted eye.
Mercury-Regulus Conjunction
After greatest elongation, Mercury begins to recede into brighter twilight. It passes 0.5° to Regulus’ upper left on the 9th. The separation is about the diameter of the Full moon. They are over 7° above the east-northeast horizon at 45 minutes before the day breaks.
Afterward, Mercury, continuing to brighten, is lower in the sky each morning, while Regulus gains altitude – more height above the horizon. On the 14th, almost rivaling Sirius’ brightness, but dimmed by the obscuring atmosphere, Mercury is about 5° above the horizon and 8.1° to Regulus’ lower left. Use a binocular to initially see the innermost planet.
The planet’s last date of visibility depends on local circumstances, such as weather and obstacles that block the planet’s view.
Mercury Moves into Evening Sky
Mercury disappears into bright twilight as it zips toward its superior conjunction on September 30th, followed by an appearance in the evening sky near Venus.
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