On May 4, just after sundown, Mercury will be just above the West-Northwest horizon. It will be a very bright Magnitude of -0.8. Just a couple of degrees north of Mercury, the Pleiades (also known as the Seven Sisters) will be dim, but visible. The Pleiades will be easier to see with binoculars. As the month proceeds, Venus will be joining Mercury in the early evening sky. They will be closest on May 17.
Mercury is the closest planet to the sun and can never be far away from the sun in the sky. Mercury is limited to a maximum height of 28˚ above the horizon. It can only be visible in the Western sky in the early evening or the Eastern sky just before dawn. In May 2021, Mercury will be higher in the sky each night until the middle of the month. It will then appear to reverse its path and will be lower each night.
Mercury is the smallest planet in our solar system, with a radius of about 1,500 miles. Jupiter’s moon, Ganymede, and Saturn’s moon, Titan, are larger than Mercury. It circles the sun every 88 days. Because its orbit is inside of Earth’s orbit around the sun, we can observe phases just like those we observe for Venus and our Moon.
Mercury is the second most dense planet in the solar system. Earth is slightly more dense than Mercury. Scientists theorize that Mercury has a solid inner core and a liquid outer core. The liquid outer core provides Mercury with a magnetosphere that can deflect the solar winds. The spacecraft Mariner 10 measured the magnetic field strength to be about 1.1% the strength of Earth’s magnetic field strength.
Mercury is tidally locked with the sun in a 3:2 Spin-Orbit Resonance, meaning for a reference to fixed stars Mercury rotates on its axis three times for every 2 revolutions around the sun. However, for an observer on Mercury, one day would be two revolutions around the sun or 2 Mercurian years. Mercury’s surface temperatures range from -280 ˚F on the dark side to 800 ˚F on the lighted side.
Mercury’s surface looks a lot like the surface of our Moon with crater and areas of past volcanic activity. Water is believed to be on the floors of deep craters at the poles which are never exposed to direct sunlight.