THE SOUTHWORTH PLANETARIUM
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Altitude: 10 feet below sea level
Founded January 1970
Julian Date: 2459218.18
2020-2021: LXIV
THE DAILY ASTRONOMER
Monday, January 4, 2021
January 2021 Night Sky Calendar Part II
*Welcome back! * We hope you have recovered sufficiently from your revels
-or, at least, from the virtual revels you attended remotely- prior to the
beginning of a new remote astronomy course. Of course, you will have
two more days to convalesce because we will first be offering the two part
January 2021 Night Sky Calendar before starting the "Exploratory
Planetarium." At or near the beginning of each month, the DA will
post a monthly sky calendar. This calendar includes what we believe to be
the month's most interesting celestial events. Due to the length, we
have separated this calendar into two parts. This month, the split will
be uneven: fewer days in Part I than II due to the greater amount of
information contained within the former section.
*SATURDAY, JANUARY 2: EARTH AT PERIHELION*
We are now moving away from the Sun. On January 2, Earth was at
perihelion, its point of least distance from the Sun. Throughout the
year, our planet travels along an elliptical, not circular orbit. What's
the difference?
- A* circle *is a closed curve consisting of points equidistant from
the center.
- An* ellipse* is a closed curve inside of which one finds two foci.
The straight line distance from one focus to any point on the curve and
then to the other focus is a constant. If the two foci occupy the same
point, the ellipse becomes a circle.
If Earth's orbit were circular, the planet's distance from the Sun would be
constant. However, as it travels along an ellipse, Earth's distance varies
continually throughout the year. Earth reaches perihelion in early
January and aphelion (the point of greatest distance) in early July.
[image:
orbital-and-rotational-characteristics-of-earth_02.jpg]
On January 2nd, Earth's distance from the Sun was 0.983257 AU, or 147.1
kilometers. [*AU *= Astronomical unit. Defined as Earth's average
distance from the Sun, an astronomical unit precisely equals 149,597,870.7
kilometers. (92,955,806 miles) . ]
The distance difference between perihelion and aphelion amounts to
approximately three million miles. How much does that change affect the
weather? Astronomers define the "solar constant" as the intensity of
solar radiation striking Earth's atmosphere. Measured in units of
kiloWatts per square meter, the solar constant's average value is 1377
kW/m^2. However, as we can see in the graph below, the solar constant
value changes in response to Earth's varying heliocentric distance:
[image: earth_sun.jpg]
Although the solar constant is reduced when the Sun is at aphelion and
increased when it is at perihelion, the change is not enough to affect the
weather.
Earth will slowly move away from the Sun until the planet reaches aphelion
on July 5 at 6;27 p.m.
*SUNDAY, JANUARY 3: QUADRANTID METEOR SHOWER PEAKS*
Well, yes, we understand that you are receiving this article on the day
after the peak. However, as the Quadrantid Meteor Shower is active
between December 28 - January 12, many of its meteors can still be viewed
during the next week. Also, we can't in good conscience neglect the
year's first major meteor shower. If nothing else, it presents us with a
perfect opportunity to discuss the science of meteor showers.
First,* meteors *are not objects. Instead, they are the lights produced
when *meteoroids,* pieces of cometary or asteroidal flotsam, infiltrate the
atmosphere. The rapidly descending particle excites the atmospheric atoms
within its vicinity. Electrons within an excited atom are elevated to
higher energy levels. The electrons then emit light when they return to
their original energy state. We perceive these light emissions as
meteors.
While we can see one meteor every 6 - 10 minutes on average, during certain
times of year, we experience meteor showers. These showers occur when
Earth passes through a stream of particles emitted by either a comet or an
asteroid. between December 28 - January 12, Earth moves through debris
cast off by 2003 EH1. Though described as an asteroid, 2003 EH1 could well
be an *extinct comet*, defined as a comet that no longer sports tails when
passing close to the Sun.
Having an extinct comet as the parent body of the Quadrantids is quite
appropriate, as the constellation for which the meteor shower is named is
likewise extinct. *Quadrans Muralis, *created by French astronomer Jerome
LaLande (1732-1807), depicted a wall mounted quadrant similar to the one he
used to develop celestial charts.
[image: Quadrans_muralis_map.png]
Although Johannes Bode (1747-1826) included the pattern in his 1801
Uranometria, the International Astronomical Union did not include Quadrans
Muralis in its list of 88 official constellations. Consequently, the
constellation is no longer recognized. The Quadrans Muralis region is
around the constellation now designated as Bootes. The *radiant*, the
point from which the meteors appear to originate, is seen in the image
below. Bootes is the sheepherder located just off the Big Dipper's Handle.
[image: 11jan03_430.jpg]
While the peak stream (the densest part through which we pass during the
peak) is thin, we can still see about 15 - 25 meteors an hour from the
Quadrantids over the course the next few nights. The best time to watch
for the Quadrantids is after midnight, the time when our part of Earth is
turning into the meteoroid stream.
Now that we've started 2021, we'll list this year's major meteor showers.
We'll experience quite a gap between the Quadrantids and the Lyrids and
another between the Eta Aquarids and Perseids. However, one can observe
sporadic meteors on any night of the year.
- *Quadrantids * December 28 - January 12; Peak Jan 3
- *Lyrids * April 16 - 30; Peak April 21
- *Eta Aquarids * April 19 - May 28; Peak May 5
- *Perseids * July 17 - August 26; Peak August 11
- *Draconids *October 6 - 10; Peak October 8
- *Orionids *October 2 - November 7; October 21
- *Leonids * November 6 - 30; Peak November 17
- *Geminids *December 4 - 17 ; Peak December 14
- *Ursids *December 17 - 26; Peak December 22
*MONDAY, JANUARY 4: LATEST SUNRISE*
Yes, it is true: the daylight duration has been increasing since the
winter solstice. On December 21st, the Sun was above the horizon for 8
hours and 55 minutes. On January 4th, the Sun will be above the horizon
for 9 hours and 3 minutes. However, the date of the earliest sunset and
latest sunrise do not occur on the solstice. At this latitude, the
earliest sunset date is December 9th; the latest sunrise date is January
4th. The sun will rise at 7:15 a.m. today. From now until mid June,
the Sun rise times will decrease.
*WEDNESDAY, JANUARY 6: LAST QUARTER MOON*
Find the last quarter moon within the constellation Virgo the Maiden
tonight.
[image: skychart.png]
*SATURDAY, JANUARY 9: MOON AT PERIGEE*
Since Earth was at perihelion earlier this month, it seems appropriate to
mention that the moon will be at perigee (its closest distance from Earth)
today. The moon will be 367,387 kilometers from Earth at this
perihelion. Like Earth and the other planets, the moon travels along
an elliptical orbit so that its distance from our planet continually varies
from its least distance (perigee) to its greatest distance (apogee). The
moon will be at apogee on January 21st.
Part II tomorrow.
It is a pleasure to see you all in class again!
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