Luke A Vavra Rare Maps And Books

LATITUDE: Latitude has been understood and measured by various means for more than 2000 years.   It has been known for about 800 years that latitude in the Northern Hemisphere is simply (but not precisely) the vertical angle from the horizon at sea to Polaris (the North Star). But, that is changing. This currently small difference is due to Earth’s axis of rotation which wobbles over the course of about 26,000 years or one degree every 72 years. During the 4th to 2nd millennium BCE (about the time the Egyptian pyramids were being built), the star Thuban in the constellation Draco (The Dragon) was the pole star. Today, it is Polaris. Thousands of years from now, the axis of rotation will point to other stars, but will return to Polaris around the year 27,800. This same phenomenon (precession of the axis) means that during the next 10,500 years our summers and winters will gradually become reversed.

Many of you have heard of the old Roman city of Mainz, Germany. An old marker showing 50 degrees north latitude is located in the heart of the old city. Late 1500s’ maps of Germany locate Mainz at 50 degrees latitude. If you’re ever in Mainz, you might come across the marker indicating this significant line of latitude! Today, with the aid of modern devises, the latitude is actually 49° 59′ 31″ North.

LONGITUDE: Measurement of longitude is another matter. Determination of longitude at sea requires accurate locations of reference points (planets, moon and stars), which are in constant relative motion, and a clock. To calculate longitude a navigator needs to compare time on his ship to the time at a port or observatory where longitude is known and he needs an ephemeris, a book that gives the locations of celestial bodies at specific times.  Until John Harrison invented an accurate marine chronometer in the mid-1700s, it was impossible to determine accurate longitude at sea or even at a newly discovered land. Earlier clocks would not work properly on a moving ship, so early navigators had to estimate the longitude using the noon-day sun and an ephemeris. This accounts for some of the huge east or west distortions seen on early maps of the Americas.

EPHEMERIS:  Historically, an ephemeris was a printed table giving the calculated positions of celestial objects at regular intervals of date and time. Ephemerides were used by early navigators to determine their positions at sea based the angles to the Sun, North Star, moon and planets. Modern ephemerides are often provided in electronic form; however, printed ephemerides are still produced.  Their use was not for the mathematically-challenged. The Global Positioning System (GPS) now obviates the need for such ephemerides except as may be needed for backup.

Regiomontanus (Johannes Müller von Königsberg), whose ephemeris was used by Columbus, was a mathematician, astrologer and astronomer. In the decades following his death in 1476, his contributions were instrumental in the development of Copernican heliocentrism where planets rotate about the Sun, as opposed to Earth as earlier thought.