[image: 8044.jpg] *Fare thee well, Arecibo...wherever you are* THE SOUTHWORTH PLANETARIUM 207-780-4249 www.usm.maine.edu/planet <http://www.google.com/url?q=http%3A%2F%2Fwww.usm.maine.edu%2Fplanet&sa=D&sntz=1&usg=AFQjCNHulkHuLP13bOG2PkNrPazsGWFs2A> 70 Falmouth Street Portland, Maine 04103 43.6667° N 70.2667° W Altitude: 10 feet below sea level Founded January 1970 Julian Date: 2459185.16 2020-2021: LI Well, Heavens! Due to memory limitations, we will apparently have to divide the December 2020 Night Sky Calendar into three sections! We will post part three tomorrow. THE DAILY ASTRONOMER Tuesday, December 1, 2020 December 2020 Night Sky Calendar Part II We'll begin part II of the December 2020 Night Sky Calendar with something that we should have included in Part I. *WEDNESDAY, DECEMBER 9: DATE OF EARLIEST SUNSET* Although the duration of daylight is shortest on and around the winter (December) solstice, the dates of earliest sunset do not correspond precisely with the solstice. In fact, these dates vary with latitude. As we can see in the graphic below, the date increases with increasing distance from the Equator. (Note: the daylight duration on the equator is 12 hours every day of the year.) While the earliest sunset in Miami occurs on November 30th, the earliest sunset in Southern Maine occurs on December 9th. The sunset time on 12/9 in Portland is 4:04 p.m. The sunset time remains the same for the next few days until December 14th, when the sun sets at 4:05 p.m. During the intervening dates, the sun set time increases by a matter of less than 10 seconds each day. [image: earliest-sunset-stephen-aman.jpg] One might wonder: if the shortest "day" is on the winter solstice, how can the earliest sunrise occur earlier? It's simple. The sunrise times will continue to increase until early January. On January 2nd, the sun rises at 7:15 a.m. in Portland. For the next few days, the sun rises at 7:15 a.m. On January 6th, the Sun will rise at 7:14 a.m. For mid-latitude Northern Hemisphere observers, the sequence is as follows: - *Earliest sunset*: early/mid December - *Shortest day:* winter (December) solstice - *Latest sunrise*: early January For mid-latitude Southern Hemisphere observers, the sequence is: - *Earliest sunrise:* early/mid December - *Longest day:* summer (December) solstice - *Latest sunset: * early/mid January The latest sunrise, earliest sunset and winter solstice would occur on the same day if Earth's orbit were perfectly circular and if the planet were not "tilted" relative to the plane of that orbit. However, because Earth's orbit is elliptical, its distance from the Sun and, consequently, its orbital velocity, is never constant. When Earth is closest to the Sun (Northern Hemisphere winter), it appears to move more quickly along the sky than it does when further away (Northern Hemisphere Summer). Also, the tilted axis draws the Sun along different arcs throughout the year, so its pathway will vary. If you're eager for longer days, take solace on December 9th. After that date, the Sun will start setting progressively later each day. *MONDAY, DECEMBER 14: TOTAL SOLAR ECLIPSE* (Northern Hemisphere observers will not see this event.) [image: SE2020Dec14T.png] Total solar eclipses are such spectacular events that they should never be omitted from any night sky calendar. As we can see from the map above, this event will not be visible to us Northern Hemisphere observers. Only those observers within the thin blue pathway will watch the moon completely cover the Sun. That line extends from the Pacific into the Atlantic Ocean and slices across Chile and Argentina. Observers outside this path, but within the partial visibility region will see a partial solar eclipse. The closer the observer is to the totality path, the greater the eclipse magnitude will be. For instance, an observer standing along the line marked 0.80 would see 80% of the Sun's diameter covered by the moon. An observer at 0.60 would see the moon blocking 60% of the Sun's diameter. Total solar eclipses occur when the moon moves directly between the Sun and Earth. Eclipses don't happen every month because the moon will either be north of south of the ecliptic plane (Earth's orbital plane) when in conjunction, or new moon. Only when the moon is aligned with this orbit during new moon will a solar eclipse occur. [image: moons_1_wk1_fig027.jpg] Each solar eclipse is either followed or preceded by a lunar eclipse. During a lunar eclipse, the moon passes directly into Earth's shadow. On November 30th, a penumbral lunar eclipse occurred. During this time of eclipse, the moon passed through Earth's outer shadow. Observers in North America were able to see the entire event, weather permitting. We didn't mention that eclipse because there was little to see, apart from some subtle darkening. *THURSDAY, DECEMBER 17: DOUBLE EVENT (BRONZE EVENT!)* * - JUPITER 3 DEGREES NORTH OF THE MOON* * -SATURN 3 DEGREES NORTH OF THE MOON* Remember that Jupiter and Saturn will appear to move closer together throughout the first three weeks of December. Tonight, the two planets and the waxing crescent moon (11% illuminated) are gathered together in the western evening sky. All three will set by or before 7:30 p.m. The double-planet/moon appulse reminds us that the night sky does not reveal depth. Jupiter, Saturn and our moon appear to be nestled close. However, millions of miles separate these worlds. On December 17th: - the moon will be approximately 230,000 miles from Earth - Jupiter will be 548 million miles away - Saturn's distance from Earth will be slightly more than one billion miles! One should experience no difficulty identifying these three worlds. The moon will appear as a thin crescent. Jupiter, at magnitude -1.8, will be nine times brighter than Saturn (magnitude 0.6). *SATURDAY, DECEMBER 19, 2020: MERCURY IN SUPERIOR CONJUNCTION* Quite a long time has elapsed since we've discussed conjunctions. We'll do so now. When the right ascensions of two bodies are equal, they are said to be in *conjunction. * *________________________________________Right Ascension Review:* Let's say you're marooned on a deserted tropical island and for some reason you want to be rescued. What two pieces of information would you need to provide to someone through the satellite radio for them to pinpoint your location? Exactly. Latitude and longitude. Offering one without the other does little good. In order to locate a celestial object, one would also need two coordinates. We have already introduced "declination," the angular distance north or south of the celestial equator. Declination is the celestial equivalent to latitude, Now, we introduce the celestial coordinate that is equivalent to longitude: right ascension. Right ascension measures a celestial object's angular distance from the vernal equinox, the point where the ecliptic intersects the celestial equator on the first day of spring. While we measure declination in degrees, right ascension is measured in "hours, minutes and seconds." Any object along the arc corresponding to the vernal equinox has a right ascension of 0 hours. The range is 0 - 24 hours. However, 0 and 24 are technically the same point. [image: celestialtime.gif] We see the right ascension as a circle running along the celestial sphere. Here again we're pretending Earth occupies the center of the Universe. The 0 h marks the vernal equinox, the point the Sun appears to occupy on the first day of spring. The 6 h mark corresponds to the point the Sun occupies on the first day of summer. The 12 hour point marks the Sun on the first day of autumn; 18 hours corresponds to the Sun's position on the first day of winter. ____________________________________________________ A planet is in conjunction with the Sun when its right ascension is the same as the Sun's. Or, when it is either directly in front of the Sun (*inferior conjunction*) or directly behind the Sun (*superior conjunction*). [image: vt2004-if11-fig4.jpg] All the other planets can pass into superior conjunction. However, only the two inferior planets, Mercury and Venus, can pass through inferior conjunction. *MONDAY, DECEMBER 21: WINTER SOLSTICE (GOLD EVENT!!!)* [image: solstice.webp] At 8:30 a.m. on Monday morning, astronomical winter will begin in the Northern Hemisphere. To explain why winter is starting now and not on December 1st as the meteorologists assert, we need to discuss Earth's annual motion. Earth revolves around the Sun once a year. As it is tilted on its axis 23.5 from vertical, the northern hemisphere is alternately pointed toward and then away from the Sun during each revolution. Winter begins when Earth's north pole is directed away from the Sun as much as possible. Summer begins when the north pole is aligned toward the Sun by the greatest amount. [image: winter-solstice-graphic.jpg] The duration of daylight will increase from now until the Summer (June) Solstice on June 20th. Happy Winter! *MONDAY, DECEMBER 21: CLOSEST JUPITER-SATURN CONJUNCTION IN NEARLY 400 YEARS! (PLATINUM EVENT!!!!) * On the night of December 21st, we command you to venture outside to look into the western evening sky. There you will behold Jupiter and Saturn about 0.1 degree from each other, an angular distance equal to 1/5th that of the full moon! This is the first Jupiter-Saturn conjunction since the year 2000 and the closest Jupiter-Saturn conjunction since 1623! To put that year in historical context, the first Shakespearean folio was published in that year. Also, 1623 was only fourteen years after Galileo first used the telescope for astronomical observation. However, that conjunction occurs 13 degrees east of the Sun, so close as to be barely visible. Only those observers who just happened to be looking into the west after sunset might have glimpsed these two worlds before they set. The closest OBSERVABLE Jupiter-Saturn conjunction prior to this event occurred in 1226! In that year, Francis of Assisi died. In the previous year, the Magna Carta was reissued for the third time in definitive form. Conjunctions of Jupiter and Saturn, also known as *Great Conjunctions*, occur every 19.6 years. They are so infrequent because Jupiter's orbital period is 11.9 years, while Saturn requires 29.5 years to complete one orbit. Nearly twelve years after this conjunction, Jupiter will have completed one revolution around the Sun. However, during that time Saturn will have completed about 40% of its orbit. Jupiter will require more than seven years to catch up to Saturn. The next great conjunctions occur on: - October 31, 2040 - April 7, 2060 - March 15, 2080 - September 18, 2100 Jupiter and Saturn will be only 6 arc-minutes apart at their closest approach on December 21, 2020. The planets will again appear this close during the March 15, 2080 event. So , let's hope for clear skies on the solstice! *MONDAY, DECEMBER 21: FIRST QUARTER MOON* The winter solstice, an usually close Jupiter-Saturn conjunction and the first quarter moon all on one night! *WEDNESDAY, DECEMBER 23: MARS SIX DEGREES NORTH OF THE MOON* [image: skychart.png] We'll see the waxing gibbous moon (66% illuminated) six degrees south of Mars tonight. Both bodies will appear to occupy the constellation Pisces the Fish. The third -and final, we think- part of the December 2020 calendar to post tomorrow. 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