If there ever was a planet that has gotten a foul rap for its incapability to be readily noticed it must be Mercury, identified in some circles because the “elusive planet.”
Mercury is named an “inferior planet” as a result of its orbit is nearer to the solar than the Earth’s. Due to this fact, it all the time seems from our vantage level to be in the identical common route because the solar. So close to to the solar that it’s usually tough to see with the unaided eye. In actual fact, Copernicus complained that he had by no means been in a position to get pleasure from a view of it, and this was likely as a result of he lived at Frombork in northern Poland, close to the Vistula River, the place the sky close to the horizon is commonly hazy owing to native mists and fog.
Rise to prominence
On Nov. 20, Mercury was at inferior conjunction with the sun, passing roughly between the sun and our Earth. Four days later on Nov. 24, Mercury passed just one degree north of brilliant Venus, but the pair were only 10 degrees from the sun and rising about 50 minutes before sunrise, shining at a magnitude of +2.4. Mercury was still impossible to see through the brightness of dawn.
But only three days later on Thanksgiving Day (Nov. 27), Mercury was rising 75 minutes before the sun and increased by a factor of 3.6 times in brightness to magnitude +1.0. It was then fairly easy to locate, close to the east-southeast horizon about an hour before sunrise.
Mercury’s visibility continued to improve rapidly and by Friday morning, Dec. 5 it will rise shortly before morning twilight begins — in a dark sky — and will have brightened markedly to magnitude -0.3. Now all you have to do is just look low above the east-southeast horizon from 40 to 80 minutes before sunrise for a bright yellowish-orange “star.”
An unusually favorable greatest elongation occurs on Sunday, Dec. 7, even though Mercury is only 21 degrees from the sun. At magnitude -0.4 (among the stars only Sirius and Canopus are brighter), it rises in a dark pre-twilight sky about one hour and 50 minutes before the sun. In the 2025 Observer’s Handbook of the Royal Astronomical Society of Canada, Alister Ling stresses that this would be the “finest morning apparition of 2025 for Northern Hemisphere observers.”
Mercury, like Venus, seems to undergo phases just like the moon. On Thanksgiving Day, Mercury was a slender crescent, 20 p.c illuminated by the solar. By Dec. 7, it’ll seem 62 p.c illuminated and the quantity of its floor illuminated by the solar will proceed to extend within the days to come back. So, though it’ll start to show again towards the solar’s neighborhood after Dec. 7, it’ll brighten a bit extra to magnitude -0.5 by Dec. 9, which ought to assist hold it in straightforward view over the subsequent couple of weeks.
Mercury passes 5.5 levels to the higher left of 1st-magnitude Antares on Dec. 19 however that ruddy star will solely shine about one-quarter as brilliant as Mercury, so you will in all probability want binoculars to identify it. Mercury itself will turn out to be tough to see in brilliant twilight by round Christmas.
The circumstances that make it occur
There are 4 explanation why Mercury will probably be so favorably positioned for viewing within the morning sky this month:
- At dawn in autumn, the ecliptic — the apparent path the sun, moon and planets take across the sky over the course of a year — makes a steeper-than-average angle with the horizon for Northern Hemisphere observers.
- Since Mercury passed the ascending node of its orbit on November 18th, it is north of the ecliptic through much of December.
- In addition, its orbital speed is near maximum, since perihelion (its closest point in its orbit to the sun) occurred on Nov. 23.
- Around the time of inferior conjunction, Mercury is much closer to the Earth, and its angular motion relative to the sun is much greater than around superior conjunction (when it’s on the far side of the sun as seen from Earth).
Exceedingly hot … frigidly cold
In old Roman legends, Mercury was the swift-footed messenger of the gods. The planet is well named for it is the closest planet to the sun and the swiftest of the sun’s family, averaging about 30 miles per second; making its yearly journey in only 88 Earth days. Interestingly, the time it takes Mercury to rotate once on its axis is 58.7 days, so that all parts of its surface experiences periods of intense heat and extreme cold. Although its mean distance from the sun is only 36 million miles, Mercury experiences by far the greatest range of temperatures: 790º F (420° C) on its day side; -270º F (-170° C) on its night side.
Planet with a double identity
In the pre-Christian era, this planet actually had two names, as it was not realized it could alternately appear on one side of the Sun and then the other. Mercury was called Mercury (Latin Mercurius) when within the night sky, however was generally known as Apollo when it appeared within the morning. It’s mentioned that Pythagoras, concerning the fifth century B.C., identified that they have been one and the identical.
Joe Rao serves as an teacher and visitor lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope and different publications.
