Archive for the ‘astronomy’ Category
How is it possible to see stars in the daytime from the depths of a well?
How is it possible to see stars in the daytime from the depths of a well? asks Balaji, via email.
In Charles Dickens’ popular novel “The Pickwick Papers,” first published as a serial in 1836 and 1837, we hear about law clerks working in a very unpleasant office:
“In the ground-floor front of a dingy house, at the very farthest end of Freeman’s Court, Cornhill, sat the four clerks of Messrs. Dodson & Fogg….catching as favourable glimpses of heaven’s light and heaven’s sun, in the course of their daily labours, as a man might hope to do, were he placed at the bottom of a reasonably deep well; and without the opportunity of perceiving the stars in the day-time, which the latter secluded situation affords.”
In other words, if these poor clerks must work in a “dark, mouldy” room behind a high partition during the sunlit hours, they should at least be able to see stars during the daytime – as someone at the bottom of a real well might. Dickens’ sentiment should resonate with any office worker trapped in a dingy cubicle (or student in a dim classroom) on a sunny afternoon.
When the Moon is just a crescent, how come we can faintly see the darkened side?
When the Moon is just a crescent, how come we can faintly see the darkened side? asks a reader.
It all starts with the Sun. The Sun makes its own glow, by fusing hydrogen atoms into helium atoms, releasing photons of light in the process. Presto: Sunshine. The Moon is lit up by sunlight, reflected back to our eyes. Voila: Moonlight-or Moonshine. But what’s the night side of the Moon lit by, since it’s turned away from the blazing Sun?
The surprising answer: Earthshine.
Walk outdoors on a sunny day into the bright light. Just as sunlight lights up the moon, it lights up the Earth. Likewise, sunlight reflects off the Earth, and some of it is reflected to the Moon. The shadowed side of the Moon is softly illuminated by the light of planet Earth. The Earth is the Moon’s own night light.
So when we look at a crescent moon, our eyes receive both reflected sunlight (from a sliver of the bright side) and Earthlight (from the darkened side). Of course, the Earthlight streaming into our eyes from the Moon is just doubly-reflected sunlight, accounting for its dim glow. One way or another, stars are ultimately responsible for lighting up their solar systems.