(Originally published in the Las Cruces Sun News on August 9, 2020 – link)
This July we received the visit of Neowise, the brightest comet since Hale-Bopp in 1997, co-discovered by the Alamogordo-based astronomer Alan Hale. Shining bright throughout July, the comet is still visible between the Big Dipper and the star Arcturus, albeit only with the aid of binoculars. Racing away from us at thousands of miles per hour, the comet will not return until the year 8786, six millennia from now.
A “great comet”, a bright enough comet to generate wide interest outside astronomy, happens in average about once a decade or two. Comets are so named because they were originally described as “long-haired stars” (from the Greek work “coma” for hair), referring to the long tail that they cast. But what are comets, and why do they arise such fascination among astronomers and the public alike?
Regular stargazers can count on the constancy of the night sky. Night after night, the stars are always at the same position, with the same brightness. Watch the sky often enough and you may come to get as familiarized with them as you are with the streets of your neighborhood. The sheer constancy of the night sky is the reason why we are so fascinated with phenomena that break this constancy: planets, variable stars, novae, meteors… and comets.
Ancient humans, in their blissful ignorance about the Universe, imagined all sorts of attributes to comets. In Ancient China, comets were thought to signify an imbalance of yin and yang. In Europe, comets were seen as bad omens, indicating calamities such as war, fires, earthquakes, plagues, famines, epidemics, or the death of a king. The apparition of a comet in 912 AD appears in the historical records of medieval Ireland: “A dark and rainy year. A comet appeared”.
The long Western tradition of seeing comets as harbingers of doom was not seriously challenged until the late 17th century when Edmond Halley proposed that the comet of 1531, 1607, and 1682 were the same comet, correctly predicting it would return in 1758, since then known as Halley’s comet. Halley’s comet, incidentally, is the same comet that the Irish attributed to their dark and rainy year of 912.
Modern astronomy understands comets as the leftover bricks that were not used to build the planets. They are small chunks of ice and rock, a few miles across, in deep freeze in the outer confines of the solar system. Astronomers describe them as “dirty snowballs”. Eventually, one of them gets too close to the Sun, and the ices boil off the surface. Because the comet itself is so small, its gravity is puny and the gases escape into space, leading to a large halo of diffuse gases, called a “coma”, that sunlight then shapes into a tail. They are classified as either short-period comets, like Halley (returning every 200 years or less), or long-period comets like Neowise (returning less frequently than 200 years). In the early solar system, when comets zig-zagged more frequently than they do today, cometary impacts were partly responsible for the delivery of water to the young Earth, making its oceans and thus possibilitating the origin of life.
Comets are also responsible for meteor showers, another beautiful show that marvels people around the world. The tail of a comet is composed of gas and dust grains that are small enough to be pushed by sunlight, but the comet also leaves behind dust debris that are too large to be shaped into a tail. If the Earth intersects the orbital path of the comet, these debris plunge into our atmosphere, and are seen as a meteor shower. The Perseid meteor shower in August are the debris in the orbit of comet Swift-Tuttle. Halley’s comet is responsible for the Orionid meteor shower in October.
So, if you missed Neowise last month and don’t have a pair of binoculars to look for it, meteor showers are an interesting way that comets can indirectly dazzle your eyes this August. The Perseids peak on Aug 12. Other meteor showers this month and visible from New Mexico are the K-Cygnids, peaking on Aug 17 (originating from an unknown comet that left the debris ages ago), and the Aurigids, peaking on Aug 31st (originating from an unknown comet that left the debris ages ago). For the Perseids look to the west of Cassiopeia, the recognizable “W” pattern of stars near the Big Dipper. For the Aurigids look around the star Capella, and for the K-Cygnids, look around Deneb. Look up, and clear skies!