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the earth is spinning around and orbiting our sun
along with a handful of planets and a few bajillion asteroids

our sun is spinning around and orbiting our galactic center
along with billions and billions of other stars
and clouds of ash dust and gas

our galaxy is spinning around too
along with billions and billions of other galaxies

how do you get your bearings when everything is moving ?
what is your reference?


the planet earth is roughly a sphere
~6,371,000m from the center to its surface

the earth is spinning about an axis
imagined from the north to the south pole

halfway between the poles
around the center of the sphere
is the equator

the angle from the equator to the poles is measured as latitude -
lines of equal latitude circle the earth parallel to the equator

the equator holds the reference position for 0� latitude
making the poles each 90� away from the equator
northward representing positive angles, southward negative angles

meridian lines are drawn from pole to pole
the angle between meridians is measured as longitude -

an arbitrary meridian is chosen as the prime meridian
which holds the reference position for 0� longitude
the prime meridian on earth passes thru a point in greenwich

the surface of the earth is full of hills and valleys
the height of a point is referenced by
the distance above or below the sphere

the height of the sphere is referenced to mean sea level
a common point where all water would come to rest
due to having equal forces of gravity
too sleepy right now to make this any cleaer, brb


the earth is slightly fatter than it is tall
not by much but enough that it can be measured

this 'imperfect' sphere is referred to as a spheroid

shperoids are defined by the radius of the axes at their maximum and minumum lengths:

semi-major axis (a)
semi-minor axis (b)

how 'flat' or 'eccentric' a spheroid is can be compared to other spheroids to clarify their differences


this image is exaggerated but illustrates that
the earth is slightly fatter than it is tall

wgs defined radii

from the center of the earth to the surface is roughly: (semi-major axis 'a') 6,378,137.0m at the equator (semi-minor axis 'b') 6,356,752.3m at the poles

| Name          | Semi-major | Semi-minor      | Datum     |
| Sphere      | 6370997    | 6370997         | Geographic|
| Clarke 1866| 6378206.4  | 6356583.8         | NAD27     |
| WGS72      | 6378135    | 6356750.519915  | WGS72     |
| ATS77      | 6378135.0  | 6356750.305     | ATS77     |
| GRS80      | 6378137    | 6356752.3141    | NAD83     |
| WGS84      | 6378137       | 6356752.3142    | WGS84     |


the surface of the earth has many features that are missed by assuming it's perfectly smooth

some ellipsoids can align with hills such that water would be running to a higher elevation

a closer approximation to the actual shape is a geoid there are different geoids made with varying accuracy this is an ongoing measurement