In a recent blog post, Karrie Berglund talked about how astronomy is a gateway science that can spark student interest in other disciplines, both scientific and non-scientific. I want to expand on this and discuss how astronomy, as the first science practiced by humans, served as the gateway science worldwide. Observing, measuring, and predicting are the basics of all sciences. At numerous archaeological sites around the world there is evidence that some of the first things humans observed, measured, and predicted were the movements of the stars, Sun, Moon, and planets.
Astronomy: Gateway to Time
The earliest known observatory sites include Taosi in Northern China dated at 2300 BCE and Stonehenge in England dating to 3000 BCE. While these sites are separated by thousands of miles and hundreds of years, both were used to tell time by tracking the Sun across the sky, and stone markers were used to mark the position of the Sun at the solstices and equinox. These markers allowed people to accurately measure and predict the seasons and to act as a basic calendar. As we learned more about astronomy we saw more and more how predictable events were. Generations of Chinese astronomers developed systems allowing them to accurately predict not just the coming of the seasons but the conjunction of planets (conjunctions are when two objects appear close to one another in the sky), and eclipses of the Moon and Sun. The invention of the telescope and the discoveries it ushered in gave us another opportunity to tell time using the heavens. When Galileo discovered four moons orbiting Jupiter in 1610, he also noticed that their orbits around the planet were measurable and predictable.
I was writing and researching this blog post about Astronomy as a Gateway Science during the time of the great conjunction of Jupiter and Saturn in December of 2020. Having many friends with telescopes I saw a large number of photographs of Jupiter and Saturn. One particularly striking picture had both Jupiter and Saturn in the frame as well as Jupiter’s four Galilean moons. It was similar to the image shown here that was captured from my Digitarium SkyBox. Seeing those moons had me thinking about the Longitude Act of 1714, celestial navigation, and telling time using Jupiter’s moons. For those of you unfamiliar with it, the Longitude Act of 1714 was an 18th Century version of the X-Prize. It offered a £20,000 prize to anyone who could come up with a method of determining longitude at sea to an accuracy of ½ degree. The problem was one of telling time: To determine longitude, you need to be able to observe a star cross the meridian, precisely know the time at your location, and know what time that star crosses the meridian in Greenwich, England. By converting the difference in hours into degrees you’ve calculated your longitude, how many degrees east or west you are from the Prime Meridian.At the time one method of telling time was to observe Jupiter’s moons. Large books called Ephemeris were printed with charts detailing the predicted positions of those moons. All a navigator needed to do to know the time in Greenwich was find Jupiter in his telescope, observe the position of the moons, and reference the Ephemeris. All in a vessel at sea. The eventual solution was a mechanical watch.
The stars and planets in the heavens prove reassuringly predictable. By carefully observing and recording these movements, humans across the globe have fashioned calendars as well as navigated across vast deserts and seemingly endless seas. But sometimes the sky could surprise, and a new star would appear.
Guest Stars and Broom Stars
In November, 1572 a new star appeared in the night sky in the constellation Cassiopeia. This was observed by a 26-year-old Dutch man by the name of Tycho Brahe. This Nova Stella (New Star) was visible during the day for two weeks and in the night sky for several months. Tycho’s careful observations showed that this star was farther away than the moon or the planets, that it must be in what until this time had been considered the unchanging realm of the stars. Tycho was not the only observer of this or other similar “stars.” In 1054 CE–500 years before Tycho Brahe’s Nova Stella—astronomers in China began observing something similar. That object remained in the sky for a remarkable 642 days. The Chinese referred to these as “Guest Stars.” The appearance of the star in 1572 inspired the young Tycho to devote his life to the observation of the heavens. He spent years plotting the movement of the stars, planets, Moon, and Sun. His observations of the planets were so accurate that after his death his assistant Johannes Kepler would use those figures to both place the Sun at its place at the center of the Solar System and to discover the laws of planetary motion. Kepler’s laws are used today to accurately predict the positions of the planets, and allow us to send spacecraft to those other worlds.
Astronomy: Gateway to History
In the year 2016 Kepler’s laws of planetary motion entered popular culture in the form of the Academy Award nominated movie, “Hidden Figures.” The movie, based on the book of the same name, follows three African-American women mathematicians (Katherine Johnson, Dorothy Vaughan, and Mary Jackson) who worked for NASA starting in the 1950s. At the time NASA used human “computers” to manually calculate flight paths and analyze and plot data. The film’s protagonist is Katherine Johnson who calculated the orbital parameters for the flights of Alan Shepard and John Glenn. The film and the book use the background of space exploration and astronomy to explore the history of racial and sexual discrimination that Johnson and her contemporaries needed to overcome.
Astronomy: Gateway to Wonder
In 1577, five years after Tycho’s new star, another guest star appeared that was dramatically different from the previous. This guest star had a long tail streaming behind it, a type of guest star Chinese astronomers called a “Broom Star.” The ancient Greeks also were familiar with these stars and called them Komitis (κομήτης) which translates as a head with long hair. Tycho measured and recorded this guest star as well, and his measurements showed that it was in the celestial realm. This was an astounding discovery that was met with some derision as most people of his time thought comets appeared in the atmosphere.
While not fully understood in 1577, comets were well known and had appeared in art for centuries. Among the first renditions were “broom star” depictions by Chinese astronomers. They also appeared in Western art: the depiction of Halley’s Comet above the Battle of Hastings in the Bayeux Tapestry is one of the best known representations.
Astronomy: Gateway to the Universe
Let’s return for a moment to Tycho Brahe and his new star. Today astronomers have determined that what was recorded in 1572 was a supernova explosion, the death of a star. More accurately, the 1572 supernova was the death of two stars, a white dwarf and a nearby companion star. This type of supernova is called a type 1a, and it occurs when the strong gravitational force of the white dwarf pulls material (mostly hydrogen and helium) from its companion. The material accretes onto the white dwarf until a violent explosion is triggered destroying both the white dwarf and the companion. This type of supernova releases a predictable amount of energy, a predictable light curve, and a predictable brightness. Today, when this type of supernova is observed, astronomers can determine how far away it is from us, by observing how bright it appears to us here on Earth. This is one of the methods that can be used to determine the vast scale of the universe. Astronomy is woven throughout human history. Astronomy’s capacity to encourage exploration, inspire wonder, and spark questions makes it a true gateway science.