The Difference a Day Makes

 

Last time, I explained how the movement of the Earth around the Sun creates the illusion that the Sun is moving through the zodiacal constellations.  There’s just one loose end to tie up.  At sunset, which is, as we’ve discovered, a good time to visual local motions, the Earth is moving down and to the right of the Sun; that is, the Earth is moving in the direction of a point below the western horizon.  That Earthly motion is what pushes the Sun constantly up and to the left.  Below, I’ve included the first figure from the last blog just to reinforce this point.  See how the Earth’s movement pushes it continuously to the right of the Sun?   And how that motion shifts the Sun continuously leftward?

This “cause and effect” is something to be savored, partly because you can think of it happening at any time of the day or night.  For example, remember from the “Beatles” blog that after sunset, the Earth’s rotation “pulls” the Sun along that line below your feet towards the Eastern horizon, on a course that will culminate in sunrise?  The Earth doesn’t stop orbiting the Sun just because it’s nighttime, of course, so throughout the night the Earth’s movement is constantly pushing the Sun back and to the left.  This has the effect of slowing the Sun’s forward progress to the eastern horizon, slightly delaying sunrise.  In other words, the Earth’s movement makes the nights just a little bit longer than if the Earth was stationary.  Similarly, the constant forcing of the Sun to the left during the day works to slow down the Sun’s forward motion then too, stretching out the length of the day.  The impact is small, adding only about two minutes of length to both day and night.   Still, it’s interesting to sit back and consider that each 24-hour day would be about four minutes shorter than we measure it to be if the Earth wasn’t moving around the Sun.

Now, let’s get back to how the Earth’s movement impacts our view of the stars.  I’ll ask you to once again imagine you’re out and looking up right at sunset, when the sun is barely above the western horizon.  If, every day, the sun is drifting slowly leftward, then every successive day that the sun sets, the stars that, the night before, were just far enough left of the Sun to be visible for a brief period just after sundown, before they rotated out of view, will now be swallowed up in the Sun’s glare, and disappear completely from the evening sky.  Meanwhile, in the morning sky, prior to sunrise, just the opposite will happen.  Since the Sun is drifting leftward, there’s a little sliver of sky visible just before dawn that was swallowed up in the Sun’s glare the night before.  Thus, in the early morning hours, just before sunrise, the Earth’s movement giveth; in the early evening, it taketh away.

On the scale of a single day, these effects are miniscule.  Over time, though, the evening losses and the morning gains add up until, over the course of a month, an entirely new zodiacal constellation becomes visible in the pre-dawn sky, while another zodiacal constellation disappears from the sky after sunset.  In any six-month period, these effects are enough to completely alter the pattern of stars that are visible overnight.

And that’s really all there is to it.  We’re done with our treatment of local motion, and I will wish you a brief adieu while I work on the next topic.  Until next time, Whabbloggers!