How Lunar Eclipses Work: A Quick Primer
A lunar eclipse occurs when the Sun, Earth, and Moon align in a straight line — or nearly so — with Earth in the middle. The Moon passes through Earth's shadow, which has two distinct parts: the penumbra (a light, outer shadow where only part of the Sun's light is blocked) and the umbra (the dark, inner shadow where the Sun is completely hidden by Earth).
Lunar eclipses can only occur during the Full Moon phase, when the Moon is opposite the Sun in our sky. Not every Full Moon produces an eclipse because the Moon's orbit is tilted by about 5 degrees relative to Earth's orbital plane (the ecliptic). When the Moon passes through the ecliptic plane during a Full Moon, an eclipse occurs. This is why we get two to four lunar eclipses per year, rather than one every month.
The partial phase of the eclipse begins when the Moon first enters the umbra. At this point, a dark "bite" appears on the lunar disc — this is the curved edge of Earth's shadow, and its shape was used by ancient Greek astronomers to deduce that Earth is spherical. As the Moon moves deeper into the umbra, the bite grows until, in a total eclipse, the entire disc is engulfed. In a partial eclipse like August 28's event, the Moon never fully enters the umbra — 96% of it does, leaving a brilliant silver sliver along one edge as a reminder of the direct sunlight still reaching the lunar surface.
Why the Moon Turns Orange
The orange-red colour of an eclipsed Moon is the result of Rayleigh scattering — the same physical process that causes sunsets to appear red. As sunlight passes through Earth's atmosphere, blue light is scattered away by air molecules, while red and orange wavelengths pass through more directly. During a lunar eclipse, the only sunlight reaching the Moon is the light that grazes Earth's atmosphere — essentially, the combined light of all the sunsets and sunrises happening around Earth at that moment. This filtered red light illuminates the Moon, giving it its characteristic copper or blood-red colour.
The exact shade depends on the condition of Earth's atmosphere. A clear, clean atmosphere produces a bright orange eclipse (L=3–4 on the Danjon scale). A dusty or volcanic atmosphere produces a darker, deeper red eclipse (L=1–2). For the August 2026 eclipse, forecasters will have a much better idea of the expected colour in the days just before the event — so check observing forecasts closer to the date. Observers should note the colour carefully and compare it to the Danjon scale — this is a scientifically useful observation that anyone can contribute.
