Spacecraft Eclipse Duration Calculator

What It Does

The Eclipse Duration Calculator returns the maximum umbra eclipse time for an orbit at a given beta angle. Input altitude, inclination, and beta angle (or date), and the calculator returns the eclipse fraction of the orbit period in seconds and as a percentage. It supports both circular and elliptical orbits and accounts for the geometric umbra cone (atmospheric refraction not modeled — appropriate for thermal and power sizing, not for star tracker operations).

Key Features

• Maximum umbra eclipse duration from orbit elements and beta angle
• Eclipse percentage of orbit period (key input for battery DOD calculations)
• Support for elliptical orbits with eclipse-near-perigee handling
• Annual eclipse envelope from orbit elements and date range

Why It Matters

Eclipse duration directly sizes the spacecraft battery. A LEO satellite spends roughly 35 minutes per 90-minute orbit in eclipse, which means batteries must support full bus loads for that duration with margin for end-of-life capacity loss. Underestimating eclipse duration means undersized batteries that brown out at solstice; overestimating means kilograms of unused mass that could have been payload. The calculator gives the worst-case eclipse for the worst-case beta angle so battery sizing has the right anchor.

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