At its brightest, Mercury can reach magnitude -1.9, a little brighter than Sirius, the brightest star in the sky. However, it more normally hovers at around magnitude 0, becoming roughly the fifth brightest object in the sky.
Nonetheless, it is always a challenging object to observe because it never ventures far from the Sun – at most 28.3°. This means it must always be observed in twilight, either at dusk or dawn. Additionally, because it is always close to the horizon at these times, it must always be observed through thick layers of the atmosphere where seeing conditions are poor.
Viewing of Mercury from the Pacific Northwest will require unobstructed views of the horizon at either sunset or sunrise as those will be the best times to see the planet. Right before sunrise or sunset is going to be the best times.
Ancient Astronomical Observations
The earliest known recorded observations of Mercury are from the Mul.Apin tablets, which were recorded on cuneiform tablets most likely made by an Assyrian astronomer around the 14th century BC. There are Babylonian records of Mercury date back to the 1st millennium BC. The Babylonians called the planet Nabu after the messenger to the gods in their mythology.
The planet was known by several different names by ancient societies. By about 350 BC, the ancient Greeks identified the planet by its twinkling and fleeting motion. The Romans named the planet after the swift-footed Roman messenger god, Mercury , which they equated with the Greek Hermes, because it moves across the sky faster than any other planet. The astronomical symbol for Mercury is a stylized version of Hermes’ caduceus.
The Greco-Egyptian astronomer Ptolemy wrote about the possibility of planetary transits across the face of the Sun in his work Planetary Hypotheses. He suggested that no transits had been observed either because planets such as Mercury were too small to see, or because the transits were too infrequent.
In ancient China, Mercury was known as “the Hour Star”. It was associated with the direction north and the phase of water in the Five Phases system of metaphysics. Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the “water star” based on the Five elements. Hindu mythology used the name Budha for Mercury, and this god was thought to preside over Wednesday. The god Odin (or Woden) of Germanic paganism was associated with the planet Mercury and Wednesday. The Maya may have represented Mercury as an owl (or possibly four owls; two for the morning aspect and two for the evening) that served as a messenger to the underworld.
In medieval Islamic astronomy, the Andalusian astronomer Abū Ishāq Ibrāhīm al-Zarqālī in the 11th century described the deferent of Mercury’s geocentric orbit as being oval, like an egg or a pignon, although this insight did not influence his astronomical theory or his astronomical calculations. In the 12th century, Ibn Bajjah observed “two planets as black spots on the face of the Sun”, which was later suggested as the transit of Mercury and/or Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.
The first telescopic observations of Mercury were made by Galileo in the early 17th century. Although he observed phases when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury.
In 1631, Pierre Gassendi made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639, Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun. A rare event in astronomy is the passage of one planet in front of another (occultation), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737 is the only one historically observed, having been seen by John Bevis at the Royal Greenwich Observatory. In 1800, Johann Schröter made observations of surface features, claiming to have observed 12 mile high mountains. Friedrich Bessel used Schröter’s drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°.
In the 1880s, Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury’s rotational period was 88 days, the same as its orbital period due to tidal locking. The effort to map the surface of Mercury was continued by Eugenios Antoniadi, who published a book in 1934 that included both maps and his own observations.
In June 1962, Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences, led by Vladimir Kotelnikov, became the first to bounce a radar signal off Mercury and receive it, starting radar observations of the planet. Three years later, radar observations by Americans Gordon H. Pettengill and Rolf B. Dyce, using the 300-meter Arecibo Observatory radio telescope in Puerto Rico, showed conclusively that the planet’s rotational period was about 59 days.
Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury’s orbital period, and proposed that the planet’s orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.