Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 3: Mapping the Earth and Sky

Key Ideas

Location? Location? Location?

Fundamental questions of geography:: 1. Where am I?; 2. Where is somewhere else?; 3. How do I get there?
Ancient maps gave distances and directions from a specific place (e.g. Rome): Fine for a flat Earth approximation; Not as good on a sphere

Dividing the Earth

Latitude and Longitude

Lost and Found

The system of Latitude and Longitude was introduced by Claudius Ptolemy (circa 140 AD)
It was forgotten in Europe after the collapse of the Roman Empire:: Flat Earth maps through the Middle Ages.; ''T-O'' maps centered on Jerusalem
Ptolemy was rediscovered, with the Spherical Earth, about 1300 AD:: The Prime Meridian goes through Greenwich; In Ptolemy's time it went through the Fortunate Isles (Canaries), which were the western limit of the known world.

The Prime Meridian

The International Meridian Conference of 1885 established:: An initial (or Prime) meridian shall be established at Greenwich; The Universal day shall be the mean solar day; The mean solar day shall begin at midnight, both at sea and on land; 22-1 vote in favor; (San Domingo opposed, France, Brazil abstain)

Celestial Sphere

The stars are projected onto a Celestial Sphere centered on the Earth
Celestial Equator:: Projection of Earth's Equator onto the sky
Celestial Poles:: Intersection of Earth's Poles with the sky
Celestial Meridian:: Great Circle from the NCP to the SCP through a given object
Declination:: Angle along Celestial Meridian from Celestial Equator to the object; Celestial Equivalent of Latitude; Measured in degrees

Celestial Longitude?

The Celestial equivalent of the Prime Meridian is not simply the projection of the Prime Meridian onto the sky
This is because it would not stay fixed due to the rotation of the Earth
We will discuss Right Ascension, the celestial equivalent of longitude, in a later lecture.

The Local Sky

Standing on the Earth, we can only see half of the sky at any instant:: One half stretches overhead to the Horizon; Other half is below the Horizon
Zenith: Point directly overhead.
Nadir: Point opposite the Zenith, below you
Cardinal Points: North, South, East, and West
Meridian: Runs North-South through Zenith

The Local Sky (cont'd)

What part of the Celestial Sphere you can see depends on: Where you are on the Earth (Latitude and Longitude); What time it is (date and time)
Effect: objects in the sky: Rise above the Eastern Horizon and; Set below the Western Horizon
as the Earth rotates.

Celestial Navigation

The Angle between the North Star and Horizon (when pointing North) is your Latitude!
In principle, if you can measure the altitude of Polaris, you are measuring your Latitude.
Variants on this are used with other stars for Celestial Naviation to determine Latitude
Longitude depends on the time, and is much harder to measure (for another day)

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.