March 2025

March 07, 2025 Posted by: Rich Fefferman

   

 

Our Sky Ranger caught Mercury and Venus shining in the west on the evening of February 28. Mercury is enhanced by the time exposure and does not look this bright in the real sky.

NPS Photo/Richard Fefferman

The planets Mercury and Venus are closer to the Sun than we are, and thus they present themselves differently in our night skies. The ancients noticed that both planets shine for up to a few hours in the western sky after sunset, then are become briefly invisible. They then emerge into view in the eastern sky before sunrise for some weeks or months before disappearing briefly once again. We now understand that these disappearances happen when Mercury and Venus pass between the Earth and Sun or appear beyond the Sun on the far side of it.

It takes Mercury just 88 days to orbit the Sun and the Earth 365, so after one Mercury orbit, the Earth will have gone only about one fourth of the way around its path. It takes quick Mercury another 28 days to catch up to Earth, so its synodic period is 116 days. Thus there are a little over three cycles per Earth year. The first part of March is actually the best chance to catch Mercury as an evening star this year, with an added benefit- brilliant Venus will be there to help you find it!

Since Venus’ 224-day orbit around the Sun is a closer match to Earth’s, its synodic period is a much longer 584 days. The Maya civilization developed a calendar around this cycle, which I wrote about in my January 2022 blog entry, which you can read about here. This March marks the end of the old Venus cycle, with a new one about to begin when Venus goes through inferior conjunction (most between Earth and Sun) on the 23rd.

 

What does this all mean? You probably have noticed Venus shining brilliantly in the western sky for the last few months. Since the end of February, Mercury has been glimmering below it, as shown in the picture below, that I took on February 28th. Venus shines much more brightly than Mercury in our skies, due to its being more than twice as big, much closer to Earth, and its cloudy atmosphere reflecting the Sun’s light very well. Mercury is also usually diminished more by the twilight glow. It is helpful to bring binoculars along to help glimpse the innermost planet, especially early in twilight or if the night is not very clear. I could see it fairly easily with my unaided eyes back on the 28th, and it should be at its best for 3-4 days centered around March 7th. During this time Mercury will fade steadily as it presents its lit side towards the Sun and away from Earth. By mid-March it will be too low and faint to see.

 

Face west about 6:45 pm St. Louis time on March 5th (above left) and 7:45 pm on the 12th (above) to spot Venus and Mercury. Note Mercury’s fading and Venus’ drop in sky altitude during this time. Both will be gone in another few days as they pass between Earth and Sun.

Courtesy of Stellarium.

Venus’ similar departure will be much more dramatic, since it shines so much more brightly. It will seem to drop noticeably every night, setting about two hours after sunset on the night of the 7th . This will be reduced to just one hour after sunset by the 16th. Within a few more days, Venus will be lost to view, depending upon how open a horizon you have, the clarity of the night, and whether or not you bring binoculars to help. You might notice Venus’ ever-thinner crescent if you can prop up your binoculars on something to steady them, or if you have a small telescope. By March 31st, try looking due east about 6:15 am St. Louis, time, a half hour before sunrise. Can you spot Venus glimmering very low in the morning twilight, marking the start of its new cycle?

 

Jupiter and Mars shine brightly in the southern sky this March. Uranus can be found in binoculars but can be a bit hard to identify amidst the starry background. To use this picture as a guide, turn it so that Orion’s orientation matches the sky.

NPS Photos/Richard Fefferman

Although the inner planets have all the drama this month, Mars and Jupiter also shine brightly. Jupiter is the bright object high in the south in the early evening, with Mars notable due to its orange color, high in the east, looking about as bright as the brighter stars in the sky. With binoculars you can even use the picture as a guide to try to identify distant Uranus.