- 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.)
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.
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.
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.
