Nodes are the points where an orbit crosses a plane, such as a satellite crossing the Earth's equatorial plane. The longitude of the ascending node is the node's celestial longitude. Celestial longitude is analogous to longitude on Earth and is measured in degrees counter-clockwise from zero with zero longitude being in the direction of the vernal equinox. In general, three observations of an object in orbit are required to calculate the six orbital elements. Two other quantities often used to describe orbits are period and true anomaly.
Period is the length of time required for a satellite to complete one orbit.
True anomaly is the angular distance of a point in an orbit past the point of periapsis, measured in degrees. Types Of Orbits For a spacecraft to achieve earth orbit, it must be launched to an elevation above the Earth's atmosphere and accelerated to orbital velocity. The most energy efficient orbit, that is one that requires the least amount of propellant, is a direct low inclination orbit. To achieve such an orbit, a spacecraft is launched in an eastward direction from a site near the Earth's equator. The advantage being that the rotational speed of the Earth contributes to the spacecraft's final orbital speed.