In the following parts of this article series, we will continue our tour of our solar system and visit all of our eight planets, including the dwarf planet Pluto. The planets of our solar system can be divided into two types, the 'rocky planets' (Mercury, Venus, Earth, and Mars) and the 'gas giants' (Jupiter, Saturn, Uranus, and Neptune). Mankind knew the existence of four planets from ancient times, that is Mercury, Venus, Mars and Saturn. The Earth was not considered a planet until the advent of the Renaissance with the Copernican model of the solar system (see Part 3 of the article series).

The name ‘planet’ comes from the Greek word planao and means “to wander” or “to go astray”. The Greeks were the first to describe in detail the movement of such “wanderers”. However the Greeks were not the first to observ the planets. The story begins as early as civilization itself.

Mercury

We begin our tour with the closest planet to the Sun, Mercury. Mercury holds two more titles other than being the closest planet to the Sun. Mercury is also the smallest planet of our solar system (This is however only after the planet Pluto lost it's status as a planet and became a 'Dwarf Planet') and the fastest to orbit around the Sun. Mercury is only 4,879 km or 3,032 miles in diameter. This makes Mercury just a little bigger than our Moon and smaller than the satellites Ganymede (moon of Jupiter) and Titan (moon of Saturn).

Figure 1: Mercury is the closest planet tou our Sun.

Although small Mercury is bright when viewed from Earth, Mercury's apparent magnitude ranges from −2.0 to 5.5 in apparent magnitude. The apparent magnitude of a celestial object is a measure of how bright it appears at the sky. Over the times astronomers agreed on a scale which defines the brightness of the star Vega as magnitude 0. Vega, pronounced veiɡa, is the fifth brightest star in the sky, so magnitude 0 is quite bright. From this point, objects can be brighter, i.e. they have negative magnitude or they can be dimmer, i.e. they have positive magnitude. This is similar to a temperature scale. However, the object which is one magnitude brighter than Vega, i.e. magnitude -1, is actually 2.512 times brighter. So, a difference in one magnitude makes an object a little more than 2.5 times brighter or dimmer. In comparison, Mercury's brightest moments in the sky makes it a little more than 12 times brighter than the star Vega.

The distance between the Sun and Mercury is in average 58 million km (36 million miles) (see Figure 1). Since the orbit of Mercury is eccentric, i.e. off-center from the Sun, Mercury is closest to the Sun at its perihelion (from Greek peri meaning “beside, close, near” and Greek helios meaning “sun”) with just 46 million km (28 million miles) and farthest away at its aphelion (from Greek apo meaning “opposite, away from” and Greek helios meaning “sun”) with 69.8 km (43 million miles). In comparitive distance, Sun to Earth is nearly 150 million kilometers or 93 million miles and is called 'Astronomical Unit (AU)'. Earth is in a comfortable distance from Sun, which makes the Earth not too hot nor too cold, but just right. Mercury, however, orbits the Sun in just one-third of an Astronomical Unit. This makes Mercury an extreme place of temperatures. An astronaut standing on the surface of Mercury would experience temperatures ranging from about −180 to 430 °C (-290 to 800 °F). This is not a nice place do be!

Figure 2: Mercury's orbit around the Sun and its 3:2 orbital resonance .

Orbiting the Sun in just 88 days this makes a 'Mercury year,' roughly as long as one Earth season. Our astronaut on Mercury has a short year, but in exchange, our astronaut gets a long day's work. Every planet rotates around its own axis like a toy pegtop or spinning top. Our Earth rotates in just 23 hours and 56 minutes. Mercury, however, rotates in 58.65 days around its own axis. The fast orbital speed of Mercury combined with its very slow rotation around its axis makes a day on Mercury 176 days! This means that Mercury rotates three times for every two orbits, known as a 3:2 orbital resonance (see Figure 2). What would you choose if you needed vacation on Mercury? I would take a day off!

Our three times title holder is not easy to observe in the night sky. Due its close distance to the Sun, Mercury is usually shows up very close to the Sun in the sky. This means, that Mercury can only be observed around 2 hours after sunset or 2 hours before sunrise. The sky will never be very dark and observation has to be done in the in the twilight sky. Due to the thicker atmosphere near the horizon, Mercury does to appear crisp and clear through an Earth-bound telescope (see Figure 3). The greatest angular separation from the Sun (greatest elongation) is only 28.3°. This is the angel between the Sun and Mercury measured from an observer on Earth at 28.3°.

Figure 3: Mercury appears hazy through an Earth-bound telescope.

Mercury in history

We have early records of Mercury from Mesopotamia from the (Greek meaning "The land between the two rivers"), i.e. Euphrates and Tigris. This region is usually referred as the 'Cradle of civilization'. Many civilizations originated there such as the Sumer as well as the Akkadian, Babylonian and Assyrian Empires, spanning from 7000 B.C. To 400 B.C. The Sumer named the Sun, the Moon and the visible planets, including Mercury, after their greatest seven gods, their Panthenon, meaning “All the Gods” (From Greek pantos meaning “altogether” and theos meaning “God”). From ancient surviving cuneiform tablets, archaeologists learned that Mercury was called by many names. However, Mercury was often associated with the deity Nabu, the god of water and writing.

The Babylonians named Mercury after their divinity Nebo or Nabu, the record keeper, god of writing, and messenger to the gods. In Babylonian astronomy, Mercury was associated with both sexes because of its appearance as both an evening and a morning star.

The Mayan culture charted the motion of the planet Mercury as well. One particularly important source is the Dresden Codex. This codex contains records of their detailed observations of the stars and planets. The codex mentions Mercury as a morning star in 733 B.C. and as an evening star in 727. Astonishingly, the Mayans also calculated that Mercury would rise and set in the same place in the sky every 2,200 days.

Figure 4: Thoth, in one of his forms as an ibis-headed man.

Ancient Egypt called Mercury 'Thoth', the great measurer, a divinity associated with knowledge and the inventor of speech, writing, and arithmetic (see Figure 4). Thoth was an important deity in the Egyptian pantheon and was often depicted with the head of an Ibis.

Figure 5: Hermes Logos is a statue of Hermes (National Museum of Rome), ca. late 1st century to early 2nd century AD.

Classical Greek around 450 B.C. started studying the motions of the planets and used geometry to measure the size of the Earth, Sun and Moon (see part 2 and 3 of this article series). Like in the case of the Mayan culture, the Greek distinguished between the morning and evening appearance of Mercury and therefore by two different names associated with its evening and morning appearances; Apollo as a "morning star", i.e. low above the eastern horizon before sunrise, and Hermes as an evening star, i.e. low above the western horizon after sunset (see Figure 5).

In ancient Indian culture the Hindus referred to the planets collectively as Navagraha, I.e. t he configuration of nine idols or the nine principal heavenly bodies. They are considered different aspects of Paramaathma or soul. Each of them represent different qualities. Mercury represents 'wisdom'. Mercury was called Budha.

Figure 6: Nebuchadnezzar II (ca. 630-562 BC)

The bible does not directly refer to Mercury as a planet. However two passages in the bible make references to other deities. In the old testament in Isaiah 46:1 the bible mentions "Nebo" who is Nabu the son of Marduk, also know as Bel the scribe god. As we saw earlier, this identifies with the planet Mercury. In the new testament, in Acts 14:11-12 the text says, "When the crowds saw what Paul had done, they shouted in the Lycaonian language,'The Gods have come down to us in human form!', Barnabas they called Zeus, and Paul they called Hermes, because he was the chief speaker” (NRSV). According to to this passage, Paul is thought to be Hermes who is identified with the planet Mercury. Other passages in the bible refer to the Nabû-kudurri-uṣur or better known as Nebuchadnezzar II (ca. 630-562 BC), the Babylonian King during the Babylonian exile, which means "Oh God Nabu, preserve/defend my firstborn son” (see Figure 6).

Figure 7: The old Norse god Odin.

So far we've introduced the Mercury planet, looked at its basic properties and reviewed how Mercury was perceived by civilization through past millenia. Even with modern technology, Mercury still hides many of its mysteries. In the next part of this article series, we'll have a look the results of the Mariner 10 and the now ongoing MESSENGER mission as they reveal some of the secrets of the “divine messenger” Mercury. See you next time!

About the Author: Juergen Riedel was born in a small town in north Germany. He was educated in the humanist tradition and completed his Masters in Physics at the Christian Albrechts University Kiel, Germany. In 1997 he went to Nassau. He is married to the Bahamian jewelry artist Kim Riedel. Together they have two boys Judah 7 and Jasper 6. Juergen Riedel is currently a part-time science teacher at Genesis Academy in Nassau and founder and director of The Science Institute ( www.thescienceinstitute.com ) which offers an in-depth science education for children in form of workshops and special activities. He believes that bringing science to children in early age will give them an edge in the international competition for future jobs. He can be reached at jriedel@coralwave.com