28
Jul
2014

Sunrise and sunset – nature’s most beautiful illusions

Earth turns on its axis to greet the sun at sunrise each morning of the year. Credit: Bob King

Every day the sun rises, crosses the sky and sets. And it does it again and again and again like the perpetually repeating cycle of events in the movie Groundhog Day.

Except perhaps for a few remaining Flat-Earthers, we know what’s going on here. The sun’s not doing the moving. Instead, the Earth’s rotation causes the apparent motion of the sun across the sky. Yet the sense of the sun’s movement is so powerfully ingrained in our experience you might balk if I told you it’s essentially sitting still in the sky.

Every day the turning Earth causes the nearly static sun to rise in the east at sunrise and set in the west at sunset. Credit: Canadian Space Agency

For you to see a sunrise, Earth must rotate on its axis until your location faces the sun as it crests above the planet’s curvature. The following morning, when Earth rolls around after another 24 hours, the sun is very nearly in the same place in the celestial sphere as the previous morning. Once again, we see the sun ‘rise’. Ditto for the next morning and the next. It’s like turning over in your bed each and every morning and seeing your spouse in the same spot. Or very nearly.

If the Earth spun but stood in one spot never circling the sun, we would meet the rising sun at precisely the same time and place every day ad infinitum – a true Groundhog Day scenario. But the Earth orbits or revolves around the sun as surely as it rotates. Just like our daily spin, our planet’s revolution is reflected in the sun, which appears to slowly crawl across the sky, inching its way from one background zodiac constellation to the next, during the course of a year.

The orbiting and titled Earth cause slow but continuous changes in the times of sunrise and sunset during the course of a year. Credit: Thomas G. Andrews, NOAA Paleoclimatology

The ever-changing times of sunrise and sunset stem from the Earth’s orbital travels combined with the shifting seasonal tilt of the planet. From December 21 until June 21, as the amount of daylight increases in the northern hemisphere, the sun appears to travel slowly northward in the sky and we meet its welcome rays a couple minutes earlier each morning.

The sun’s yearly motion across the sky during the year traces out a path called the ecliptic. The top of the curve, at right, is the sun’s position during the summer. The low part of the curve is the sun’s location during winter. The up-and-down path is a reflection of the 23 1/2-degree tilt of the Earth’s axis. Illustration and animation by Dr. John Lucey, Durham University

Then from June 22 to December 20, Earth’s orbital motion causes the north polar axis to slowly point away from the sun. The sun appears to slide south as the hours of daylight wane, and we meet the sunrise a minute or two later each morning.

The sun, located some 26,000 light years from the center of the Milky Way galaxy, takes about 220 million years to make one revolution around its core moving at 483,000 mph. Credit: ESO

Earth moves along its orbit at an average speed of 67,000 mph (108,000 km/hr).

How about the sun? If I left the impression that it’s totally static I apologize. Yesiree, it’s moving too – at the astonishing speed of 483,000 miles per hour (792,000 km/hr) around the center of the galaxy.

Don’t look now, but you and I are going on the ride of our lives.The only reason stars remain static in the sky over the span of many generations despite the sun’s hurry is because nearly all of them are too far away to show a shift in position with the human eye. Telescopes, which magnify everything including motion, do show very subtle changes in the positions of nearby stars over much shorter time intervals.

Rising each morning to the same old sun, I try to remind myself that with every rotation comes a new opportunity to spin some joy into the day.

2 Responses

  1. Grant

    Hi, Bob:

    I’ve been searching for the answer to the following question for quite some time. Your page comes closest to answering it, but not quite.

    My question has to do with the fact that the length of days at the solstices varies by just a few seconds from day to day, but the length of days at the equinoxes varies by about 2 min, 26 sec from day to day (sample location: Raleigh, NC).

    We know that the earth’s orbit is elliptical, but just barely. It’s very close to a circle–much more than the other planets. Can the earth’s slightly elliptical orbit be the sole explanation for the almost negligible daylight differential from day to day right around Dec. 21 and June 21? Is the earth like an automobile that must slow down a little to get around a sharp curve? That doesn’t seem quite right.

    Thanks for your consideration,
    Grant

    1. Profile photo of astrobob

      Hi Grant,
      Great question! The rapid rate of daylight change is not related to the shape of Earth’s orbit, which is nearly circular as you describe. Instead, it’s because the Sun’s declination (it’s north-south movement) is changing rapidly around the time of the equinoxes. While it moves the same amount in the sky as at the solstice, most of that movement is north/south. This causes a dramatic and steady change in the length of daylight especially if you live well north or south of the equator. At the solstices, the Sun is transitioning from south to north (or north to south) and moves more east/west than north/south. Does this help?

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