[Letters] Citizens of the universe

A couple days ago, the day before the winter solstice, to be exact, I attended the closing ceremony of the “International Year of Astronomy 2009.” The year, chosen by the International Astronomical Union and Unesco for a year-long celebration of astronomy, is nearing the end, and I reflected on the past four centuries of human triumph.

Exactly 400 years ago, Galileo Galilei (1564-1642) made the first recorded observation of the moon with a telescope. It was December 1609. In Korea, this was one year before Heo Jun, a Joseon dynasty doctor, compiled a 25-volume encyclopedia on Eastern medical principles called “Donguibogam.”

Mountains and craters placed in disorderly fashion, Galileo’s observations did not come close to the contemporary image of the perfect heaven. A month later, he discovered four moons around Jupiter.

Jupiter also turned out to be a planet like the Earth. Following the discovery, Galileo confirmed that Venus repeats phases like the Moon.

In fact, Venus turned out to be orbiting the Sun rather than the Earth. The universe that Galileo saw turned out to be the display of heliocentric cosmology formulated by Copernicus in 1543.

It was also in 1609 that Johannes Kepler (1571-1630), published “Astronomia Nova.” The German mathematician and astronomer claimed that the planets move in elliptical orbits focused around the Sun, rather than in perfect circles. He also said that a line joining a planet and the Sun, during equal intervals of time, results in equal areas.

In 1618, he presented the third law of planetary motion: the square of a planet’s orbital period is proportional to the cube of the semi-major axis of its orbit. The last law is still used today in estimating the mass of a black hole.

The significance of Kepler lies in the flexible applicability of his principles. Using his laws, he estimated when Mercury and Venus would pass in front of the Sun. The actual occurrences were observed in 1630 and 1639, respectively.

Galileo dealt with the simple geometry of orbits, but Kepler went beyond this and investigated the physics involved. This is why Kepler referred to his work as the “new astronomy.”

If Galileo’s telescope played the role of an estimator by estimating the Earth’s place in space, Kepler’s laws of planetary motion opened humankind to the path toward discovering the truth of the universe.

Last March, NASA launched a spaceship named Kepler in order to discover Earth-size planets. During the next three and half years, the Kepler will exclusively monitor changes in brightness of more than 100,000 stars.

I hope that the mission will soon find Earth-like planets and declare that year of the finding as the “Cosmological Year of Astronomy.” It will be the day that we humans qualify as justified citizens of the universe.

Hong Seung-soo, Professor Emeritus

in Department of Physics and Astronomy,

Seoul National University