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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 5: Mapping Earth & Sky

Key Ideas:

Angular Units:

Terrestrial Coordinates:

Celestial Sphere:

Local Horizon & Zenith

Finding yourself...

Age-old questions of geography:
  1. Where am I?
  2. Where is someplace else?
  3. How do I get there from here?

Ancient maps usually gave locations in terms of distances and directions from a specific place (e.g., Rome or Alexandria). This is fine for a flat earth approximation, but not obviously so good on a sphere, especially when distances get large. On spheres, it is better to use angular coordinates.

Measuring Angles

The Babylonians started the tradition of dividing the circle into 360 degrees.

Start by dividing the circle into quarters (90 degrees), then subdividing further using geometric constructions.

Subdividing the Degree

Degrees are divided into Minutes of Arc ('):

Minutes are divided into Seconds of Arc ("):

Question: Why 60?

Answer: The Babylonians (again)...

60 is divisible by 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30 without fractions.
The Babylonians actually subdivided the degree as fractions of 60, for example:

7 14/60 degrees

Claudius Ptolemy introduced the modern notation of expressing angles in terms of minutes and seconds of arc

7 14' 00"

Angular Coordinates on Spheres

Since the Earth's surface is approximately spherical, we divide the surface into a grid of arcs rather than a rectangular grid.



Prime Meridian:



Relating Angles to Distances

How big is a second of arc?
We can translate differences in angles to lengths:

Distance = (angle/360 degrees) times the circumference of the Earth

One degree is about 110 km or about 68 miles.
One minute is 110 km/60 or about 2 km (~1 mile).
One second is 110 km/3600 or about 31 meters or 34 yards.

Lost & Found

The system of Latitude and Longitude was invented (or at least brought into its classic form) by Claudius Ptolemy (c 140AD), the "Father of Modern Geography".

It was all but forgotten in Europe after the collapse of the Roman Empire:

Ptolemy was rediscovered, with the Spherical Earth, about 1300:

The modern system of latitude and longitude is largely the same as Ptolemy's except for the details.

The Celestial Sphere

The Sun, Moon, and Stars are so far away, we cannot perceive their relative distances as depth in the sky. Instead, they appear to be projected onto a Celestial Sphere centered on the Earth.

Celestial Equator:

Projection of the Earth's Equator onto the sky.

Celestial North & South Poles:

Intersection of Earth's Poles with the sky

Celestial Meridian:

Great Circle passing North-South through the North Celestial Pole (NCP) and South Celestial Pole (SCP) on the sky.


The Local Sky

From any particular location on the surface of the Earth, we can only see half of the sky at any instant: In addition to the Horizon, we define a few specific points on the sky:
The point directly overhead.

The point opposite the Zenith, directly below your feet.

Cardinal ("Compass") Points:
The 4 cardinal directions: North, South, East and West.

What part of the Celestial Sphere you can see depends on

The overall effect is that we see objects rise above the Eastern Horizon, and set below the Western Horizon as the Earth Rotates.

Here we see the local sky "dome" placed on the Earth at the position of Columbus, Ohio, with the Celestial Sphere drawn. The visible half of the sky at this instant is shown in green, while the part of the sky below the horizon (and so invisible) at this instant is shown in red. As the Earth rotates towards the east, those parts of the sky just below the eastern horizon will rise in the east, while those just above the western horizon will set.

Celestial Navigation (part way)

The Angle Between the North Celestial Pole and the North Compass Point on the Horizon is your Latitude!

In principle, if you can measure the altitude of Polaris, you are measuring your Latitude to a precision of about 1 degree.

Variants on this technique are at the heart of the practice of Celestial Navigation for deterimining Latitude today.

Longitude depends on the time, and is much harder to measure (and is another story for another day).

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