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Touring the Skies By Jim Bonser (jbonser@usa.net)

June 27, 2019
Reinbeck Courier
Ps. 19:1 The heavens declare the glory of God; the skies proclaim the work of his hands.

I just love Iowa. We have four very distinct seasons: Spring which only lasts for a week or so some years but seemed to never end this year, Summer which always seems to fly by way too quickly, Fall which never fails to please as the harvest is brought in, the leaves go from luscious green to beautiful reds, yellows and browns, and Winter which I wish would fly by as quickly as Summer but it never does. I’m sure most are aware that the seasons are caused by the tilt of Earth’s axis with respect to the plane of our yearly orbit around the Sun. For the hemisphere that the pole is tilted toward the Sun, the days are longer and the Sun is higher overhead much of the day which means the Sun’s rays have longer to warm the ground and the air during the day and less time to radiate that energy away at night. When the North Pole is tilted towards the Sun, it is Summer for us and winter for the southern hemisphere. And when the South Pole is tilted towards the Sun, it is Summer for the southern hemisphere and winter for us. On June 21st we experienced the moment when the North Pole was pointed most directly towards the Sun and therefore, we enjoyed or perhaps I should say, as an amateur astronomer, endured the longest day and shortest night of the year. This month, we begin to enjoy warm but shorter days and (yay!) longer (and warmer) nights. There is one other aspect of the geometry of our orbit around the Sun that I thought you might be interested in knowing. Our orbit is very close to being a perfect circle. Almost, but not quite. The amount that the orbit of a planet deviates from being a perfect circle is called its eccentricity. Amazingly, even though it is so far away, the gravitational force of Jupiter actually causes the Earth’s orbit to vary from nearly circular with an eccentricity of 0.005 to quite elliptical with an eccentricity of 0.06. Currently, our orbit has an eccentricity of 0.0174, which is nearly its minimum. As we travel around the Sun in this less than perfect circle, at one point we are closer to the Sun (perihelion), and 6 months later we find ourselves a little farther from the Sun (aphelion). Now, guess which part of the orbit we are in this month. Ready? Did you guess perihelion – the close to the Sun part? If you did, you were wrong! The Earth will reach aphelion, the place in our orbit when we are farthest from the Sun, on July 4th! So, as you are celebrating our country’s independence from British rule this year on a hot July 4th, remember that we are actually as far away from the Sun that day as we will get this year! How far is that, you ask? Well, according to Bruce McClure at EarthSky.org, “At its closest point, (January 2nd, 2019) Earth swings to within 91,403,554 miles (147,099,761 km) of the sun. […], and when the Earth reaches aphelion – its most distant point – on July 4, 2019, then we’ll be 94,513,221 miles (152,104,285 km) from the sun.” A difference of about 3 million miles. So now you know! I took a picture of the Sun back in January. It was cloudy on the 2nd, but it cleared up a couple of days later and I got the shot. I am hoping that we will have at least partly cloudy skies on the 4th so I can take another picture using the same equipment and compare the size of the Sun at aphelion vs perihelion. If it works out, I’ll be certain to post the picture for you! Jupiter is that wildly bright “star” visible in the southeast shortly after sunset. Do make every effort to get out to your local astronomy club’s public viewing night this month so you can get a good look through a telescope. Jupiter reached opposition in June and appears very big in a telescope. You won’t be disappointed and be sure to observe Jupiter’s 4 bright moons while you’re at it! A waxing gibbous Moon will be about 4 degrees northeast of Jupiter on the 13th. Saturn reaches opposition this month on the 9th. Two days after the Moon’s close encounter with Jupiter on the 13th, an almost full Moon will visit the ringed planet. The two will be only about 2 degrees apart. Saturn will look spectacular with a telescope this month. Those beautiful rings are tipped about 24 degrees so they will take your breath away. Be sure to take advantage of the nice evening temperatures and clean, clear skies we often enjoy in July to get outside and take in the beauty of Iowa’s skies at night! Clear Skies!

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