| 60 | seconds (′′) | = | 1 minute (′) |
| 60 | minutes | = | 1 degree (°) |
| 360 | degrees | = | 1 circumference (cir.) |
| Circum- ference | Degrees | Minutes | Seconds | |||
| 1 | = | 360 | = | 21,600 | = | 1,296,000 |
A quadrant is 1⁄4 of a circumference, or 90°; a sextant is 1⁄6 of a circumference, or 60°.
The length of a degree of longitude on the earth’s surface at the Equator is 69.16 miles.
In astronomical calculation 30° are called a sign, and there are therefore 12 signs in a circle.
LONGITUDE AND TIME
The earth’s circumference (which has the form of a circle) at the equator is (3.1416 × 7926), 24900 miles; which divided by 360, gives 69.17 miles for 1 degree of longitude at the equator. Leaving the equator, degrees of longitude gradually diminish, since all meridians converge at the poles. Thus, 1 degree of longitude, at 10 degrees of latitude, is 68.1 miles; at 20 degrees 65 miles; at 30 degrees 59.9 miles; at 40 degrees 53 miles; at 50 degrees 44.5 miles; at 60 degrees 34.6 miles; at 70 degrees 23.7 miles; at 80 degrees 12 miles; at 90 degrees 0.
Imaginary lines running north and south, through these degrees, from pole to pole, are called meridians. Those east and west, parallels.
One meridian which runs through Greenwich, near London, England, is called the prime meridian, and all the other meridians are reckoned as east or west of it.
Longitude is distance east or west of the prime meridian. When we say that the longitude of [865] Paris is 2° 20′ East, we mean that the meridian running through Paris is 2° 20′ east of the prime meridian that runs through Greenwich, England. The longitude of Washington, D. C., is 77° 7′ West. That means that the meridian which passes through Washington is 77° 7′ west of the prime meridian.