Partial List of Problems an Astrolabe Can Solve

• Determine the hour and direction of the sunrise (or sunset), at a given date.
• Determine the instant where the Sun will have a given Azimuth, at a given date.
• Determine the instant where the Sun will have a given Azimuth and Altitude, without knowing the date.
• Determine the hour of rise (or set) of a star on the rete, at a given date.
• Determine the instant of culmination of a star on the rete, at a given date.
• Determine the maximum altitude of the Sun over a year for a given location.
• Determine the maximum altitude of a star at a given date, for a given location.
• Determine the hour by measuring a star altitude, for a given location, at a give date.
• Determine the right ascension and declination of a star.
• Determine the sidereal time for a given location, at a given instant.
• Determine the end of the evening twilight for a given location, at a given date.

Shout Out to Liba Taub

The curator of the Adler Planetarium’s collection of rare scientific instruments when I was there, is now at Cambridge University’s history of science museum, called the Whipple Museum. There is usually a creative activity on their website, and I recommend checking it out.

Three Problems Learned People Had With the Copernican Model

1. If the Earth actually spun on an axis (as required in a heliocentric system to explain the diurnal motion of the sky), why didn’t objects fly off the spinning Earth?
2. If the Earth was in motion around the sun, why didn’t it leave behind the birds flying in the air?
3. If the Earth were actually on an orbit around the sun, why wasn’t a parallax effect observed? That is, as illustrated in the adjacent figure, stars should appear to change their position with the respect to the other background stars as the Earth moved about its orbit, because of viewing them from a different perspective (just as viewing an object first with one eye, and then the other, causes the apparent position of the object to change with respect to the background).

The first two objections were not valid because they represent an inadequate understanding of the physics of motion that would only be corrected in the 17th century. The third objection is valid, but failed to account for what we now know to be the enormous distances to the stars.

—excerpted from class notes on the University of Tennessee’s website.

Chaucer's Treatise on the Astrolabe

The text of Chaucer's user manual for the astrolabe, written as a letter to his son, is available online at Fordham University: http://www.fordham.edu/halsall/source/chaucer-astro.asp