Astronomy of the Ancients

 

In centuries past, man was interested in the heavens for two main reasons: telling time and navigation. Creating a calendar based on the movement of the celestial bodies was essential for knowing when to plant and harvest crops, as well as knowing when hold religious festivals. For example, the ancient Egyptians were the first to establish the excellent estimate of 365 days per year in their calendar, and they did this between 5000 and 4000 BC. The annual floods of the Nile were probably the impetus for this discovery, and they soon linked this event with the day that Sirius, the Dog Star, rises in line with the Sun in the morning. The ancient Celts built Stonehenge between 2800 and 1800 BC, and it is accepted that it was used by the Druids to predict summer and winter solstices (at the very least), important times of the year in the Celtic religion. Ancient astronomers in Middle East locales like Babylonia and Sumeria made extensive observations of the sky, and so did the people of India and China. The excellent calendars of American cultures like the Mayans were based on the heavens. Ask any archaeologist and she will be hard pressed to name an ancient culture that did not have a keen interest in the Sun, Moon, and stars.

An intimate knowledge of the sky also aided maritime navigators, well away from land and any points of reference. For example Polynesian sailors, whose navigational feats were not surpassed until the invention of Global Positioning Systems, could accurately determine their position in the ocean using only the stars, allowing them to safely navigate between remote islands without getting lost while traveling across thousands of miles of open ocean in the South Pacific. It is not known exactly how they did this. Western sailors from Europe used the Sun and the Pole Star to estimate their latitude. A good familiarity with the stars also allowed sailors to make a rough guess at their longitude, as a star's position at a given night of the year will be dependent upon location of the ship. In 1731, the sextant was invented, a device which can accurately measure the angular distance between celestial objects. With this device, latitude could be determined to within a nautical mile or two, even from a swaying deck. Using a sextant, explorers could also finally calculate their longitude using the lunar-distance method, which required a book of astronomical tables, though this was a difficult process.

So, the first astronomers were farmers, sailors, and religious leaders. Not much later though, astronomy became a pursuit to gather knowledge and understand the Universe, purely to satisfy that unquenchable human curiosity. Greek astronomers started this systematic investigation, which was picked up much later in Europe. The Greek astronomer Hipparchus made many important contributions to astronomy, including the first measurement of precession, a process whereby the positions of the stars in the sky gradually change over many thousands of years. Hipparchus also compiled the first star catalog. Another Greek astronomer named Ptolemy created the geocentric (Earth-centered) model of the solar system that lasted for 1300 years until Copernicus proposed the heliocentric (Sun-centered) system during the Renaissance. Ptolemy also published a star catalog that referenced the work of Hipparchus. The Copernican revolution in astronomy was supported by Johannes Kepler who used precise naked-eye observations made by Tycho Brahe to discover the three laws of planetary motion, the Laws of Kepler. Galileo Galilei also supported the heliocentric model of the solar system, and after he constructed a 20x refractor telescope he discovered the moons of Jupiter and Sunspots. (Galileo's blindness later in his life was most likely caused by his looking at the Sun with his telescope, a very dangerous thing to do!)

Isaac Newton became the father of modern astronomy when he united physics and astronomy. He discovered that the same force that causes objects to fall on Earth also causes the motion of the planets and the Moon. Using his Law of Gravity he explained the Laws of Kepler giving the heliocentric system the backing of physics. Newton also discovered that white light from the Sun could be split into a spectrum of its component colors, a crucial basis for much of 20th century astronomical research based on spectroscopy.

 

The Celestial Sphere

Here we will cover some fundamentals of astronomy, all of which were learned by observing the heavens with the naked eye. It is easy to see why ancient astronomers placed the Earth at the center of the Universe, with everything else revolving around it. Sit outside any day or night, and you will see it's the objects in the heavens that move, and not the Earth! Well, the rotation of the Earth gives that illusion anyway, and it is still useful when trying to understand the movement of celestial bodies to imagine the Earth upon which you stand as a flat plane bisecting a sphere. All of the heavenly bodies are then drawn on the surface of the sphere, regardless of their true distance from Earth. The plane intersects with the surface of the sphere at the horizon. This celestial sphere, as it's called, has a north and south celestial pole as well as a celestial equator, which are all projected from the corresponding reference points on Earth. The celestial sphere rotates, east to west, and the stars trace arcs through the sky parallel to the celestial equator. The zenith is the point in the sky directly above you, making a 90 degree angle from the horizon. The angle between the horizon and the north celestial pole (NCP) is equal to the observer's latitude on Earth. Since the star Polaris, the so-called North Star, is almost exactly on the NCP, ancient mariners could quickly determine their latitude at night by estimating the angle between Polaris and the horizon. Conversely the angle between the zenith and the NCP is 90 degrees minus the observer's latitude, so if you happen to be standing on the Earth's equator (zero degrees latitude), the NCP will be on the north horizon and the south celestial pole (SCP) will be on the south horizon.

The daily rotation of the Earth causes each star to revolve around the north celestial pole. This is called diurnal motion, pictured below. At present, the star Polaris (the North Star) is very near the North Celestial Pole. Due the a process called precession of the equinox (a sort of "wobble" in the Earth's rotation) which has a period of about 26,000 years, the positions of the stars in the celestial sphere gradually change. Here is an image of the path of precession. In 3000 BC, the very faint star Thuban in the constellation Draco was the pole star. In about 14000 AD, the brilliant Vega will likely be the pole star. The Greek astronomer Hipparchus first estimated the Earth's precession in 130 BC. To do so, Hipparchus found a temple in Thebes, Egypt built in 3200 BC with a known orientation to the star Spica. He could then measure the change in Spica's position since the time of the temple's construction.

Just like the Earth has a latitude and longitude coordinate system, astronomers have created a coordinate system to identify the location of celestial objects on the celestial sphere. An object's declination is measured by degrees from the celestial equator, and is similar to latitude on Earth. So, an object located 10 degrees south of the celestial equator will have a declination of -10 degrees. An object 10 degrees north of the celestial equator will have a declination of +10 degrees.

Right ascension is similar to longitude on Earth, only it is measured in hours, minutes, and seconds since the Earth rotates in the same units. On Earth, the starting point for longitude is the prime meridian located in Greenwich, England. The starting point for right ascension is called the vernal equinox at zero hours right ascension, and it is the point where the ecliptic and the celestial equator intersect. When the Sun is on the vernal equinox, the hours of night and day are equal, and this typically happens sometime on March 21st. The point opposite the vernal equinox on the celestial equator is the autumnal equinox at 12 hours right ascension, and there are 24 hours to make the complete circle. The right ascension and declination coordinate system is useful because it can specify the location of a celestial object regardless of the observer's position on Earth.

Another coordinate system which is easier to understand is the Altitude-Azimuth coordinate system. The altitude is measured by degrees up from the horizon along a line toward the zenith (at 90 degrees altitude), which is a point in the sky directly overhead. A good way to estimate this is by holding your fist vertically against the sky with your arm outstretched. Your fist will cover roughly 10 degrees, so if you can fit 4 fists between the horizon and the North Star, the altitude of the North Star is roughly 40 degrees, and you have also just measured your latitude at 40 degrees north of the equator. Azimuth is also measured in degrees along the 360 degree circle of the horizon, where due north is zero degrees, due east is 90 degrees, due south is 180 degrees, and so forth. So an object at 60 degrees altitude and 90 degrees azimuth will be due east and about 6 fists up from the horizon. The disadvantage of the Altitude-Azimuth system is that the coordinates are totally dependent on your position on Earth. It is good to learn it, however, because it is easy to learn and many web sites will give you Alt-Az coordinates based upon your global position.

To estimate the right ascension and declination of objects in the sky (for the northern hemisphere), one needs some reference points. First of all, picture the celestial meridian line, an imaginary line that runs from a point due north on the horizon through the zenith to a point due south on the horizon. Right ascension coordinate estimates are based on the meridian. Astronomers use something called sidereal time which is the number of hours since the vernal equinox has crossed the observer's meridian. The sidereal time is also equal to the right ascension of the stars now crossing the meridian. Estimating right ascension is a bit tricky, but fortunately those in the northern hemisphere have a celestial clock called the Big Dipper and the North Star (Polaris). First, identify the Pointers in the Big Dipper. These are the two stars at the end of the scoop that, when an imaginary line is drawn through them, point directly at the North Star. A line from Polaris through the Pointers sweeps counterclockwise through the sky once each day. When that line is pointed upward (the Pointers are above Polaris) the sidereal time is 11 hours, since the right ascension of the pointers is almost exactly 11 hours (and the Pointers are now on the meridian). When the line from Polaris through the Pointers goes due left, toward west on the horizon, the sidereal time is 6 hours later, or 17 hours. When the Pointers are below Polaris, so the line from Polaris through the Pointers goes straight downward, the sidereal time is 23 hours. Estimating the right ascension of a star not currently on the celestial meridian can be difficult, and remember that all of the hour circles, imaginary lines for each hour of right ascension, pass through the celestial poles. You'll always have one or two hour circles as reference points: the line drawn from Polaris through the Pointers, which is the 11 hours right ascension circle, and the meridian which is the sidereal time hour circle.

A good reference point for declination is the zenith. The declination of the zenith will always be equal to the altitude of the north celestial pole (Polaris), which is also equal to the observer's latitude on Earth. A second reference point for declination can be the celestial pole itself (Polaris), which by definition has the maximum declination of 90 degrees.

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Please contact Adam if you have questions or comments about this page. Research and image sources are provided when possible.

Sources:

Images:
Images are shown here for noncommercial educational purposes.
The images of the celestial spheres and diurnal motion are from an online astronomy course Birth and Death of Stars presented by Dr. James Schombert at University of Oregon.