The Hindu’s weekly Science for All newsletter explains all things Science, without the jargon.
A rare total solar eclipse was visible across North America on April 8, casting a shadow across the U.S., Canada, and Mexico. The eclipse was visible to one of the biggest audiences of the phenomenon on the continent ever, with a couple hundred million people living in or near the shadow’s path.
The science of solar eclipse: how does it occur?
A solar eclipse occurs when the sun, moon, and earth line up, either partially or fully, and the moon passes between the earth and the sun. The moon casts a shadow on the earth, blocking out sunlight either fully or partially, causing an eclipse.
A solar eclipse is not a common occurrence because the paths of these astronomical bodies don’t often align to create the perfect conditions.
A solar eclipse can either be total, annular, or partial.
A total solar eclipse (like the one on April 8) occurs when the moon completely blocks the sun. The sky darkens, and the sun’s outer atmosphere is visible in the form of a ring of light with the moon blocking the rest of it.
An annular solar eclipse occurs when the moon passes between the sun and the earth at its farthest point from the earth. Because of its distance from the earth, the moon appears smaller and does not completely cover the sun.
A particular cosmic convenience that allows total and annular eclipses to happen is the comparable sizes of the moon and the sun in the sky. The moon appears a particular size because it’s much smaller and also much closer. The sun is much bigger but also much farther away. Their relative sizes and distances are a coincidence and they make ‘ring of fire’ eclipses possible.
A partial solar eclipse occurs when the earth, the moon, and the sun are not in perfect alignment, therefore the moon only covers a part of the sun, making it appear like a crescent from the earth.
Why can’t we observe a solar eclipse from all parts of the earth?
The main reasons that a solar eclipse is not equally visible from all parts of the earth is the planet’s curvature and tilted axis.
The earth’s surface curves away from the observer, which translates into different views of the eclipse from different points on the surface. For example, during a total solar eclipse, only the observers seeing it from within the path of totality, which is traced on the earth’s surface by the moon’s shadow, will observe complete darkness. The path is determined by the alignment of the moon, the earth and the sun, which differ due to the shapes of the moon’s and earth’s orbits as well as gravitational interactions between the bodies.
The earth takes approximately 24 hours to complete one rotation around the sun. The rotation causes different parts of the earth to face the sun at different times, causing changes in the visibility of an eclipse depending on the observer’s location. The earth’s axis is also tilted at 23.5 degrees, which affects the path of the moon’s shadow during eclipses, causing variations in visibility of the eclipse depending on the time of the year. When the tilt of the earth’s axis positions a particular hemisphere more directly towards or away from the sun, it affects the angle at which the moon’s shadow falls on the earth’s surface. This causes a shift in the path of totality during a solar eclipse.