10/25/2021 – Ephemeris – Mercury is at its greatest separation west of the Sun of the season

This is Bob Moler with Ephemeris for Monday, October 25th. Today the Sun will be up for 10 hours and 29 minutes, setting at 6:41, and it will rise tomorrow at 8:13. The Moon, 3 days before last quarter, will rise at 9:38 this evening.

This morning at 12:59 am, the planet Mercury reached its greatest western elongation from the Sun. It reached an angle of 18.4 degrees west of the Sun. It will be visible for most of the 7 am hour low in the east southeastern sky. Mercury will be visible to the naked-eye for the next week or 10 days. The planet has quite an elliptical orbit. And it happens that in our favorable appearances of Mercury on fall mornings and spring evenings, we are seeing Mercury in the same part of its orbit. Which is near its perihelion, the closest part of its orbit of the Sun. Observers in the Southern Hemisphere see Mercury’s favorable elongations near aphelion, its farthest from the Sun, of around 28 degrees separation from Earth’s viewpoint.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

Addendum

Mercury, a day after greatest western elongation at 7:54 am, or 20 minutes before sunrise tomorrow morning, October 26, 2021. Also shown is its orbit, if we could see it. Mercury is virtually in the same position this morning. Created using Stellarium.

10/22/2021 – Ephemeris 3,001st post – Mercury is now visible in the morning before sunrise

This is Bob Moler with Ephemeris for Friday, October 22nd. Today the Sun will be up for 10 hours and 38 minutes, setting at 6:46, and it will rise tomorrow at 8:09. The Moon, 2 days past full, will rise at 7:53 this evening.

The planet Mercury is now visible on clear mornings, low in the east-southeastern sky after 7 am, and twilight gets too bright around 7:50 am. Tiny Mercury is the smallest planet, only 50% larger than our Moon. It looks a lot like our Moon, close up, all gray and covered in craters. But the resemblance is only skin deep. Before the Arecibo radio telescope could bounce radar pulses from the planet in the 1960s, we thought Mercury held one side pointed at the Sun and the other side eternally away. That wasn’t the case due to its elliptical orbit. It rotates in 59 days, two-thirds of its year of 88 days. This makes its solar day, noon to noon, last two of its years. The European BepiColombo mission to orbit Mercury just made its first pass of the planet.

Addendum

Mercury and its apparent orbit for 7:49 am tomorrow, October 23, 2021, two days before its greatest western elongation (separation from the Sun). Created using Stellarium.

BepiColombo takes a picture of Mercury on its first of 6 flybys, October 1st, before settling into orbit of the planet on December 5, 2025. BepiColombo is actually two spacecraft connected together, and parts of the other spacecraft will get into each other’s images until they separate. Credit: ESA/JAXA.

11/28/2019 – Ephemeris – This is the best time to spot Mercury in the morning

Ephemeris for Thanksgiving Day, Thursday, November 28th. Today the Sun will be up for 9 hours and 8 minutes, setting at 5:05, and it will rise tomorrow at 7:57. The Moon, 2 days past new, will set at 6:59 this evening.

The planet Mercury is at its greatest western elongation from the Sun this morning. Being west of the Sun it rises first. It being a planet inside the Earth’s orbit it appears to swing from the east side to the west side of the Sun, from the evening to the morning side of the Sun. It never appears outside of twilight around here. For the next week it will be going away from us, and becoming more illuminated by the Sun from our perspective, and actually is getting brighter. Then it will seem to fall back, actually moving around and behind the Sun. This evening the thin sliver of the Moon will appear near brilliant Venus. The best time to look is around an hour after sunset, which is about 6 p.m. Venus will set at 6:43 p.m.

The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

Mercury in the morning, about an hour before sunrise (7 a.m. here) for the next week. The aspect of its orbit changes for two reasons, sunrise time is advancing a bit, and the Earth is changing its position with respect to Mercury and its orbit, and Mercury too is moving. Create using Stellarium and GIMP.

09/12/2016 – Ephemeris – Mercury passes inferior conjunction with the Sun today

Ephemeris for Monday, September 12th. The Sun will rise at 7:18. It’ll be up for 12 hours and 39 minutes, setting at 7:58. The Moon, 3 days past first quarter, will set at 3:34 tomorrow morning.

Today the Planet Mercury will pass inferior conjunction, that is move between the Earth and the Sun. Unlike last inferior conjunction, when it passed directly in front of the Sun on May 9th. This time it will pass below the Sun. The term inferior means it is between the Earth and the Sun. A superior conjunction is when Mercury passes the back side of the Sun. Mercury will be moving into the morning side of the sky, and toward the end of the month it will be much easier to spot than it was last month when it was low in the west. On autumn mornings the ecliptic, the path of the Sun and planets, sticks up, close to vertical, while in the evenings it lies close to the horizon, which is also why Venus sets so soon after the Sun now.

Times are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

Mercury last August 16 at greatest eastern elongation. Note how low Mercury is to the horizon at sunset. Created using Cartes du Ciel (Sky Charts).

Note in the charts above and below the line passing through the Sun, and the one the planets hang around is the ecliptic, the projection of the Earth’s orbit on the celestial sphere.  The grid line running through the west compass point at the horizon is the celestial equator.  I’m allowing us to see below the horizon.  As planets rise, move across the sky and set they will appear to move parallel to the celestial equator.

Mercury this coming September 28 at greatest western elongation. Note how high Mercury is to the horizon at sunrise. Created using Cartes du Ciel (Sky Charts).

Note too that Mercury appears farther from the Sun on August 16th than at September 28th.  And it is.  On August 16th Mercury appears 27.4 degrees from the Sun.  On September 28th, it’s only 17.8 degrees.  That’s a big advantage for observers in the southern hemisphere.  The best times to see an eastern elongation of Mercury is on winter and spring evenings, and the best times to see a western elongation of Mercury is on summer and autumn mornings.  Since the seasons are reversed from the northern hemisphere to the southern hemisphere.  Our not so good August 16th elongation of Mercury was a really good one for folks south of the equator.  We’ll return the favor September 28th.  But the best southern hemisphere greatest elongations are always greater in the separation of Mercury from the Sun  than northern ones because Mercury has a markedly elliptical orbit.

 

 

02/24/2015 – Ephemeris – Not all greatest elongations of Mercury are equal

Ephemeris for Tuesday, February 24th.  The sun will rise at 7:29.  It’ll be up for 10 hours and 54 minutes, setting at 6:23.   The moon, 1 day before first quarter, will set at 1:28 tomorrow morning.

Today Mercury is at its greatest distance west of the Sun.  Don’t go out to view Mercury right before sunrise unless you’re way south of here, or better yet south of the equator.  Because of the tilt of the Earth’s axis and our latitude the positions of the planets that appear near the sun at sunrise and sunset are easier or harder to spot depending on the season.  The best time to spot Mercury when it’s at its greatest elongation or distance east of the Sun is on late winter and spring evenings, or late summer and autumn mornings when Mercury is west of the Sun.  Since the seasons south of the equator are opposite ours the same rule holds.  It’s late summer down there which is favorable to spot Mercury in  the morning, but not here.

Times are for the Traverse City/Interlochen area of Michigan.  They may be different for your location.

Addendum

Mercury at greatest western elongation on late a winter morning. Mercury is off to the side of the Sun and quite low. This is a poor elongation for us in the northern hemisphere. Created using Stellarium.

Mercury at greatest western elongation on an autumn morning. Mercury is above the Sun and quite high. This is a good elongation for us in the northern hemisphere. Created using Stellarium.