When the moon obscures the sun

“The sun is like fire and the moon like water,” wrote Zhang Heng, a Chinese astronomer, in 120 AD. The analogy was used to explain one of the differences between stars and planets. The sun or fire gives out light and the moon and other planets have the nature of water so they reflect light coming from the sun.

To a certain extent, the comparison is also useful to explain another astronomical fact. From our point of view here on Earth it seems that, sometimes, the water can put out the fire. In reality, the moon is not “putting out” the sun, it is simply blocking its light during a certain amount of time. These solar eclipses happen when the sun, the Earth and the moon are aligned, with the latter located between the sun and our planet.

During a solar eclipse, the moon’s shadow darkens a portion of the surface of our planet. This shadow on the Earth consists of two regions, the umbra (or, possibly, the antumbra) and the penumbra, because the sun is not a point but an extended light source. While no direct sunlight penetrates the umbra, the penumbra is reached by light coming from part of the sun’s disk. An observer within the penumbra sees a partial solar eclipse, that is, part of the sun obscured by the moon. An individual within the umbra is presented with a more spectacular event, one in which “the water completely puts out the fire”, a total solar eclipse.

Solar eclipses are remarkable in that they happen because the moon and the sun have almost the same apparent size. Because the orbit of the moon around the Earth is an ellipse, as it is the orbit of the Earth around the sun, the apparent sizes of the sun and the moon can vary. Depending on the position of the moon in orbit, its apparent size can either be slightly larger or slightly smaller than that of the sun.

A total eclipse can only happen if the moon is in a position such that it’s apparent size is larger than that of the sun. When it is smaller, the moon’s umbral shadow does not reach the Earth. Instead, what is called the antumbra shades a portion of the Earth’s surface. Observers in the antumbra see an annular solar eclipse, a phenomenon in which the moon does not cover the totality of the sun and a “ring of fire” is visible.


Total (A), annular (B) and partial © solar eclipses (source: Wikimedia Commons).

We live in a peculiar time in the history of the Earth because we are able to see both total and annular eclipses. Many millions of years ago, the moon was closer to the Earth than it is now and had a larger apparent size resulting in total solar eclipses being more common than today. Because of tidal friction between the Earth and the moon (the same effect that causes the tides in the ocean), our satellite is slowly getting further away from our planet. As a result, in a few hundred million years, the moon as seen from the Earth will be too small to completely block sunlight and only annular and partial eclipses will be visible.

In the meantime, we will keep seeing “the water putting out the fire” in various manners. The next total and annular eclipses will occur in 2012. 2011, on the other hand, is rich in partial eclipses with the moon partially covering the sun four times this year.

Observers in Europe, parts of Africa and Asia will be able to see the first solar eclipse of the year as early as tomorrow morning. Individuals in northern Sweden are the luckiest in that they will see the greatest eclipse: at about 8:50 am, over 85% of the sun’s diameter will be blocked by the moon. For viewers in the UK, the eclipse will last roughly between 8 and 9:30 am. Weather permitting, observers in London will see the moon occluding 70% of the sun’s diameter. As mentioned on NASA’s website, it will be an “excellent opportunity to photograph the sunrise eclipse with interesting foreground scenery.”

Remember never to look at the sun directly as doing so may irreversibly damage your eyes. If you would like to watch tomorrow’s eclipse you should use a solar filter, appropriate eclipse glasses or observe the sun via a projection of its image.


3 thoughts on “When the moon obscures the sun

  1. Linda Lin

     Learned something interesting from that 🙂
    It’s so science-fiction-y to catch a solar eclipse. It’s like some giant saucer ate the sun and then it’s several minutes of night during what supposed to be day. But the reasoning’s really logical. It be would fantastic to catch that next solar eclipse. And see other astronomical things..like a midnight sun


  2. Barbara Ferreira

    I’m glad you did!
    A midnight sun would indeed be an awesome thing to see. Or an aurora. Sadly, I’ve seen none of that. Or a total solar eclipse for that matter! Typically, you have to travel to far-away places to catch a glimpse of those astronomical wonders.


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