Twice per year

-Equitorial and Orbital plane meet

March 20/21 - Spring Equinox

Sept 22/23 - Fall Equinox

Twice per year

Equitorial and Orbital plan are furthest away

June 20/21 - Summer Solstice

Dec 22/23 - Winter Solstice

Defined as the position of the sun @ the time of Vernal Equinox

Named for constellation where sun was when it occured

Kept by clocks

Assumes a circular orbit

Time for the sun to return to the same spot overhead

23 hours and 56 minutes

1. Planets orbit in ellipses with sun @ 1 focus
2. Law of Areas
   • Equal area in equal time
     • If A1=A2=A3
     • Then T1=T2=T3
3. P² α a³ - Square of Period is proportional to cube of mean distance from sun.

When m is closest to M

Velocity is greatest

When m is furthest from M

Velocity is the smallest

Force =

Gm₁m₂

r²

G=6.6742 x 10^-11

Coordinate Systems

ECI

Earth Centered Inertial

Fixed Vernal Equinox

Coordinate Systems

GCRF

Coordinate Systems

ECEF

Coordinate Systems

Perifocal Frame

Most Common

Earth Centered Orbit Based Inertial

S/C fixed - rotating

Not for circular orbits

PQW unit vectors. P points towards perigee

If outer target is <11.9x inner, H.T is better

If outer target is >15x inner, Bi-elliptical  is better