Understanding Elliptical Earth Orbits
Why an Ellipse Matters
An elliptical Earth orbit has two main distance points. Perigee is the closest point to Earth. Apogee is the farthest point from Earth. These values control the size, shape, and speed of the path. A satellite moves faster near perigee. It moves slower near apogee. This behavior comes from conservation of energy and angular momentum.
Core Orbit Measures
The semi major axis is the average of perigee radius and apogee radius. Eccentricity describes how stretched the orbit is. A value near zero means the path is almost circular. A higher value means the path is more elongated. The semi minor axis helps describe the width of the ellipse. The focus distance shows how far Earth sits from the center of the ellipse.
Velocity and Timing
The calculator uses the vis viva equation for speed. This equation links speed with radius, orbit size, and gravity. Period comes from Kepler based motion. It estimates the time needed for one complete revolution. Mean motion shows how many radians the satellite sweeps each second. The true anomaly field studies a selected point along the orbit. It gives radius, speed, and time since perigee for that point.
Practical Uses
Mission planners use these values for transfer paths and parking orbits. Students use them to check homework steps. Designers can estimate whether an orbit is low, medium, or highly elliptical. Custom gravity and radius inputs also support other Earth models. They can help compare simplified assumptions with mission notes.
Good Input Practice
Use altitude above Earth, not distance from Earth center. Keep apogee greater than or equal to perigee. Use kilometers for all distance fields. Use a positive gravitational parameter. Small rounding differences are normal. Real missions include drag, thrust, perturbations, and Earth rotation. This tool gives a clean two body estimate. It is best for learning, planning, and quick comparison. The result should be checked before any operational decision. Export options help save calculations for reports.
Reading the Output
Review the warning note first. Then compare radius, speed, and period. Check the anomaly section when studying position. Keep the exported file with assumptions. That record prevents confusion during later reviews. Repeat entries whenever assumptions or constants change.