StarWatch: Moravian College Astronomy

JANUARY 2021

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1272 JANUARY 3, 2021: Sequel to the “Star”: The great conjunction of Jupiter and Saturn that brought memories of the Star of Bethlehem into focus this past Christmas is fading, but one last saga of the story needs to be highlighted: what ensued after the triple conjunction of Jupiter and Saturn and how there is a similar grouping of planets that will be happening from January 9 through the 11th. I would also like to provide a comparison with the conjunction of today and the one of yesteryear that possibly heralded the birth of the Christ Child over 2000 years ago. There are some notable differences. Our 2020 conjunction was rare only because of the closeness that Jupiter approached Saturn (1/10th degree), and it was a relatively short event lasting throughout the latter half of December. Convergences of Jupiter and Saturn occur every 20 years. The most popular astronomical explanation for the Star of Bethlehem took about a year to transpire and began with Jupiter and Saturn emerging from the glare of the morning sun in the predawn winter months of 7 BC. Jupiter then caught up to and passed Saturn on May 29 of 7 BC, similar to what we have just witnessed. The separation of Jupiter and Saturn was one degree, 10 times greater than what has just occurred. By the summer of 7 BC, the Earth was catching up to both planets which were still near to each other in the sky. The analogy is identical to a car passing a group of trucks on a long, steep hill. During the act of driving past the trucks, the 18-wheelers appear to move backwards or to retrograde. This is exactly what occurred with Jupiter and Saturn as the faster orbiting Earth caught up to and passed the two slower orbiting Jovian worlds. This caused Saturn and then Jupiter to retrograde, to begin moving backwards in July of 7 BC. Since Jupiter was closer to the Earth, its retrograde motion was more exaggerated, and it passed Saturn while both planets were moving westward for a second time on September 30 of 7 BC. This must have piqued the Wise Men’s attention because retrograde motion was not a well understood phenomenon at that time. As the Earth passed Jupiter and Saturn, both planets assumed their normal eastward motion in the heavens, causing faster orbiting Jupiter to overtake and pass Saturn high in the evening sky for a third time on December 5 of 7 BC. If that was not enough, Mars brought a conclusion to the “Star” by entering the scene and catching up to Jupiter and Saturn to form a loose triangle in the western sky during late February of 6 BC. This brings us full circle to what will be occurring on January 9-11 when Mercury, instead of Mars, will enter the scene to form a loose triangle with Jupiter and Saturn. Mercury will be the planet to the left. Jupiter will be ahead of Saturn which will be the faintest of the three. All three planets will be visible in the same binocular field of view. To see this association, you’ll have to scout a location with an impeccable southwestern horizon. The center of the triangle will be only three degrees above the horizon, 35 minutes after sundown, so the sky will still be bright. You may catch Jupiter and Mercury with the unaided eye, but I think binoculars will be needed to reveal Saturn. While the 6 BC conjunction involving Mars would have been placed a little higher in the sky, the convergence of Mercury with Jupiter and Saturn is another similarity with respect to the “Star” which prodded the Wise Men to journey to Judea to pay homage to the Christ Child.

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JANUARY 10, 2021: Winter Halos

On December 30, I witnessed the most vibrant lunar halo (See here) of my 60+ years of observing the heavens. November 30 and January 3 were halo nights too. These types of atmospheric phenomena are more common during the winter months. They can be created by the sun or the moon and generally form at high altitudes in cirrus clouds where small, pencil-shaped (hexagonal) ice crystals are prevalent. Without dark adaptation, the red interior and blue exterior of the December halo was immediately apparent upon going out-of-doors from a brightly lit room. Part of the halo was diffuse which I first thought was due to cloud structure, but I later discovered that it was really a distinct entity called a circumscribed halo. It was also more intense than the inner, more common 22-degree halo. Also, when I digitally processed my images, an additional whitish halo became visible known as a parhelic circle. It intersected the moon and fanned away from both sides. This was not detectable as I visually observed the other two halos. • The 22-degree lunar halo and circumscribed halo are refraction phenomena caused by light entering and passing through mediums of different density, air to ice to air. Each wavelength of sunlight (color) is bent uniquely, separating the different energies to create the rainbow of colors which emerge from the crystal. Blue light is bent at a steeper angle than red light, thus causing the observer following the line of sight of these chromatic “rays” to see the blue light forming the outer regions of the halo and the red light creating the inner boundary. The interior edge of the halo represents the minimum deviation (refraction) angle (22 degrees) of red light emerging from the tumbling ice crystals, thus giving rise to its name, the 22-degree lunar or solar halo. • In order for the circumscribed halo to form, light must pass through alternate faces of hexagonal (pencil-shaped) ice crystals that are falling through relatively still air with their long axes parallel to the ground. This is analogous to the way leaves fall from a tree or what occurs to grass when thrown into tranquil air. Here is where halo research becomes mind-boggling because the circumscribed halo takes on radically different shapes for different moon and sun altitudes. You might guess that in halo research computer simulations play a primary role in understanding these shapes. I found a computer simulation for a circumscribed halo at a moon/sun altitude of 70 degrees which provided excellent agreement to my image with a moon altitude of 72.5 degrees. • That brings me to the white parhelic circle that I first thought was a lens defect. Delving into Robert Greenler’s, Rainbows, Halos, and Glories, Cambridge University Press, 1980, I found the answer. This halo was created by moonlight reflecting off the narrow sides of hexagonal plate crystals or the ends of pencil crystals. In both cases the plate surfaces and the long axes of the hexagonal crystals had to be parallel to the ground. This causes the parhelic circle to sweep around the sky parallel to the horizon and at the same altitude of the sun or moon creating it. • Besides the sheer beauty of these apparitions, everyone sees a unique halo. When my wife, Susan, joined me, a distinct set of hexagonal crystals created the halos she saw. Even the individual colors are created by different crystals, each positioned at their specific refraction angles from the moon or sun. • Halos are truly one of the most wonderful and intriguing gems of the atmospheric optics family. Keep looking up, and eventually you will be bound to get the opportunity to see this spectacular sight.

The annotated 22-degree halo of December 30, 2020 is shown after digital processing. The true size of the highly overexposed moon would be the size of one of the "Os" that spell moon. Gary A. Becker image...

The 22-degree lunar halo of December 30, 2020 was captured at 1:41 a.m. with the moon positioned at an altitude of 72.5 degrees. After digital processing the skeletal trees were added to enhance the aesthetics of the photo. Gary A. Becker image...

1274 JANUARY 17, 2021: Snapple Caps: True or False: When I taught in the Allentown School District Planetarium, now the Learning Dome, Jesse Leayman, a former student and friend, gave me a mini-fridge with a broken thermostat. Apparently when it malfunctioned, it was at its lowest setting, just at the freezing point or slightly below. I stored my diet peach Snapple in that fridge, but the liquid never froze. The bottles would sit quietly, sometimes for weeks, waiting for a quick shake or simply a pop of the cap. That would cause the water to crystallize, sometimes creating a fast-moving haze that descended down the container as the water solidified. I would consume my cold Snapple slushy with my lunch, a really great treat after three, fast-paced classes in a row. • In order for water to freeze, it needs impurities (nuclei) around which the ice crystals can form (nucleate) and lock into a rigid shape. Supercooled water can also be made to freeze if the pressure on the liquid is increased. Shaking the bottle can dredge up some of the tea leaf impurities at the bottle’s bottom while popping the cap increases the pressure on the liquid which can cause freezing to commence immediately. Snapple is bottled hot; so as the liquid cools, the remaining air space decreases in energy, reducing the pressure and creating a partial vacuum. I saw both instances with the Snapple that I consumed. • That brings me to the wonderful world of Snapple caps, another source of fun for the random information which they contain; however, are they always correct? Actually, most of the astronomical ones are good, but here is an example of a Snapple Cap (snapple.com) that was not, Number 353: The weight of the moon is 81 billion tons. No way, José: False… Weight is a force acting on a mass in a gravitational system. I weigh 175 pounds on Earth, but I would only weigh about 29 pounds on the moon. That is because the mass, the amount of matter that the moon contains, is only 1.2 percent of Earth’s mass. Surface gravity is also a function of the distance (d²-distance times distance) of an object’s location from the center of the attracting body. The Earth’s radius is four times greater than the moon’s radius, reducing Earth’s attraction to only 1/16th of what it would be if the Earth were the same size as the moon, but with an equivalent amount of matter. If the moon were gently set upon the Earth’s surface, it would have an approximate weight of 8 x 10¹⁹ tons. That is 19 zeros to the right of the eight before the decimal. If the moon were placed on the sun and did not immediately begin vaporizing, it would have a weight 28 times greater. The weight of you or me at the surface of, let’s say, a planet is really the amount of force acting upon a less massive body (you or me) at the radius of a more massive object. Eighty-one billion tons is about one billion times too small an amount. • Snapple Cap 445: If there are two full moons in a month, the second one is called a blue moon. Correct, with this addition… The current definition of a blue moon actually arose from a mistake in the March 1946 issue of Sky and Telescope magazine. The original definition came from the 1937 edition of the Maine Farmers’ Almanac. It is called the third full moon in a season where four full moons occurred, the blue moon. The last blue moon was seen on Halloween 2020. The next blue moon willhappen on August 31, 2023. More about Snapple bottle caps at some future time. Ad Astra!

1275 JANUARY 24, 2021:

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JANUARY 31, 2021: