Reading the Nodes (Time)
Interpret eclipse geometry using nodes — the points where the Moon's tilted orbit crosses Earth's orbital plane — and understand that eclipses only occur when the Moon is near a node at the time of Full or New Moon.
Key Concepts
In an evolutionary tree, the vertical position of the branching point (node) represents time .
A node near the top represents a recent split (a recent common ancestor). A node near the bottom represents an ancient split (a distant common ancestor).
Common Questions
What are nodes in the Moon's orbit?
Nodes are the two points where the Moon's tilted orbit intersects Earth's orbital plane. Only when the Moon is near a node during Full or New Moon can a lunar or solar eclipse occur.
How do nodes explain the timing of eclipses?
Eclipses require three-way alignment: Sun, Earth, and Moon must line up. Since the Moon's orbit is tilted, perfect alignment only happens near the nodes, limiting eclipses to specific windows in the lunar cycle.
Why don't eclipses happen every month even at New and Full Moon?
The Moon is at a node only twice per orbit cycle. Most Full and New Moons occur when the Moon is above or below the node, so Earth's shadow misses the Moon and the Moon's shadow misses Earth.