Several bright stars and a quartet of morning planets shine from the pre-sunrise sky.
by Jeffrey L. Hunt
The Summer Triangle – Vega, Altair and Deneb – shine from overhead this morning. This stellar trio belongs to their own constellations – Lyra, Aquila, and Cygnus, respectively.
During early summer evenings the triangle begins the nights low in the eastern sky as the sky darkens after sunset. Because darkness – the time between the end of evening twilight and the beginning of morning twilight – lasts only about 4.5 hours at the northern mid-latitudes, the stars only make it to overhead before the glow of the anticipated sunrise begins.
Lyra is a relatively small constellation. Its bright stars appear as a parallelogram attached to a triangle that includes Vega.
Bright Jupiter and Saturn are lower in the south-southwestern sky in eastern Sagittarius. Faster moving Jupiter overtakes and passes the slower moving Saturn once every 19.6 years. This once-in-a-generation event is known as a Great Conjunction. This year’s occurs on December 21. This is the closest conjunction of the two planets since 1623.
As the planets revolve around the sun, they move eastward compared to the stars. As our planet revolves around the sun, the motion of the planets in the sky reflects the combined movements of that planet and Earth. As we overtake and pass Jupiter and Saturn nearly every year, they seem to stop moving eastward compared to the stars and go backwards.
This retrograde motion was difficult to explain for the early astronomers, and the description was more difficult when our planet was placed at the center of the universe without motion. After the invention of the telescope and the earth’s revolution around the sun was confirmed, astronomers clearly understood this was an illusion of our faster moving planet overtaking and passing the outer planets.
On July 14, Earth passes between the sun and Jupiter. Earth passes Saturn six days later. This is known as opposition. The planets appear opposite the sun in the sky. They rise in the east when the sun sets in the west. They are in the south near midnight, while the sun is there near noon. Finally, they set in the west as the sun rises in the eastern sky.
Jupiter and Saturn have been retrograding since May and continue into September.
This morning Jupiter and Saturn are 5.7° apart. Until retrograde ends, the gap grows. You can watch them separate and move westward compared to the starry background. Use a binocular to locate 56 Sagittarii (56 Sgr on the image) and Sigma Capricorni (σ Cap). On the next several mornings, Jupiter and Saturn are near this morning’s locations, but a farther west (right) compared to the stars. This morning Jupiter is 1.7° below its reference star. Saturn is 2.6° to the lower right of σ Cap.
In the southeast, Mars is moving eastward compared to the stars. It moves faster than Jupiter and Saturn. Observing its changing position is easy. This morning it moved into Pisces. The constellation is without bright stars, so a binocular is handy when observing the motion of the planet. It is difficult to visualize two fish connected by a string in these faint stars.
Mars is 0.2° to the upper right of 27 Piscium (27 Psc). Watch it pass the star and move toward 29 Piscium (29 Psc) during the next few mornings.
Mars stops moving eastward on September 9 and begins to retrograde. The Red Planet is at opposition on October 13, 2020.
As the sky brightens as sunrise approaches, Venus rises in the east-northeastern sky. It is visible during bright twilight. In the image above, Venus is clearing the trees about 45 minutes before sunrise.
The five bright planets and the crescent moon appear together on the morning of July 19.
Here’s more about the planets during June.