A Sunny Slant Of View On Earth Day

A halo and circumscribed halo (upper bright saggy arc) around the sun this morning. Both are formed when light is bent or refracted through pencil-shaped hexagonal ice crystals. Photo: Bob King

Happy Earth Day! What a great planet to call home. Situated in our sun’s habitable zone and endowed with life and sweet pleasures, we know of no where else like it. I wouldn’t trade this blue gem for all the planets of Star Trek.

The sun photographed by NASA’s Solar Dynamics Observatory earlier this morning April 22. The large sunspot group at upper right can be glimpsed with the naked eye only through a safe solar filter. Click to enlarge. Credit: NASA

The sun broke through morning clouds today with two pleasant surprises – a colorful circumscribed halo caused by light refracted through billions of microscopic ice crystals in high cirrostratus clouds, and a pair of naked eye sunspots.

Through a safe solar filter, I spied two side-by-side black dots in the upper right (northwest) quarter of the sun.

High-resolution closeup of sunspot group 1726. Sunspots are cooler regions of concentrated magnetic energy on the sun’s surface. They usually have two parts – a dark, inner umbra enclosed by a lighter penumbra. Credit: NASA

They’re part of the large sunspot group 1726 that today spans more than a dozen Earth diameters (approx. 100,000 miles / 160,000 km). The group spawned a few flares, including a moderately strong M-class flare earlier this morning, and holds the potential for more.

Two cool snowpeople catch some April rays last week in Duluth, Minn. Photo: Bob King

In April the sun’s slant in the sky is the same as it is in mid-August, high enough to feel on your cheeks when you walk out the door. It brings welcome relief to some of us still mired in winter with several feet of snow piled up in the yard. The angle of the sun in the sky has much to do with the amount of heat our planet receives and the global climate.

The overhead sun covers a smaller area of ground with the maximum amount of energy (left). At right the sun is shown in winter when its lower angle spreads its light over a larger area with a loss of 100 watts of energy. Result? Colder weather. Credit: Randy Russell / windows2universe.org

If Earth were a gigantic flat disk instead of a sphere and had no atmosphere to filter sunlight, every square meter of ground would receive 1,368 watts of solar energy. Our planet’s spherical shape spreads the sun’s light out over a larger area, diluting the energy received to 342 watts. The atmosphere also filters out a small amount.

A different view of the sun’s angle in winter versus summer and its seasonal energy footprints.  Credit: Nicholas M. Short

When the sun is nearly overhead, as it is during summer, we get the full wattage and really feel the heat just like you would standing next to a 342 watt light bulb. In winter, the sun shines on the planet at a lower angle and its light spreads out broadly across the ground.

Since the amount of energy it’s beaming our way is constant, if it’s spread over a larger area, it becomes less concentrated and weaker. That’s one big reason why winter’s so cold – a lower sun means the intensity (energy) of sunlight is reduced. Without the customary heat to warm air and ground, rain becomes snow and accumulates.

The atmosphere also plays a part. In winter, the low sun shines spends much of the day shining through the lower levels of the atmosphere, where the air is thicker and dustier. Some of that light gets absorbed and some reflected away from Earth, further cooling the season.

Today’s angle suits me just fine. When I walked outside I felt the sun right away and knew it would be good day to celebrate another day of life on the planet. Wishing the same for you.

12 Responses

    1. astrobob

      Hi Carol,
      Not really. It stands 5 degrees high right at sunset. We’ll probably have to wait until mid-May to see it.

        1. astrobob

          One of my favorite astro things is watching for that first evening for Venus to re-appear in the west.

          1. caralex

            Yes, after last spring’s wonderful months-long apparition, I really missed it when it crossed the sun last June and vanished!

            May I ask another question (apologies for asking questions irrelevant to the subject of the post) about Mars?

            It’s only a few days ago that Mars went behind the sun. It seems to have been low in the west for months before that, hesitating, as it were, to get on with it and go to conjunction. What caused that long delay? I know it has to do with the speeds of both Earth and Mars in their orbits keeping them on one side of the sun, but do you have a diagram to show just how this works? I have some idea, but would love to see it explained visually. Thanks.

  1. caralex

    Oh, thanks, Bob. That’s EXACTLY what I was looking for! I knew the two planets were ‘chasing’ each other on the same side of the sun, but I needed to ‘visualize’ it to understand it to my own satisfaction. Thanks again!

  2. Sean

    Bob, it looks like in the halo photo u may also have a LOWER circumscribed halo, at the bottom of the 22-degree one (i.e., lower tangent arc). What do u think?

    1. astrobob

      Hi Sean,
      Yes, there’s definitely a little something down at the bottom, enough of an arc to brighten it up.

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