THE SOUTHWORTH PLANETARIUM
207-780-4249      www.usm.maine.edu/planet
70 Falmouth Street     Portland, Maine 04103
43.6667° N                   70.2667° W
Founded January 1970
Julian date:  2457687.16
                        "Cubs in the series?!?

THE DAILY ASTRONOMER
Monday, October 24, 2016
Week 8 Night Sky Calendar
October 23 - 29,  2016

We have a lot of ground to cover and a lot of sky to see, so this week we'll dispense with our usual gratuitous  preface and proceed headlong into our weekly night sky calendar!

TUESDAY,  OCTOBER 25:  MOON 1.5 DEGREES SSW OF REGULUS
Regulus is the brightest star in Leo the Lion.  This "little king" star represents the lion's heart and marks the southern point on its "sickle" asterism.    Tonight, one will find the waning crescent moon hovering just to the south of Regulus.     The moon rises at 2:00 a.m. tonight and will be 22% illuminated.   As the moon is not particularly bright at this illumination, one will easily be able to observe the moon and this bright star.  

​Leo the Lion.      A late winter/early spring constellation, Leo the Lion rises after midnight this time of year and remains in the eastern sky until dawn.  The image above -courtesy of space.com- shows an outline of the lion.  Regulus marks the southern tip of the lion's  The waning crescent moon will appear to pass just south of Regulus on October 25th. 

WEDNESDAY, OCTOBER 26:   VENUS 3.1^o N of ANTARES
Even though Antares is the brightest star in Scorpius the Scorpion, Venus will be 100 times brighter than Antares during this apparent close approach.     (For more information about calculating this brightness difference, please refer to "From the Catacombs of Infinite Knowledge.")      This planet and star are low in the southwestern sky and will set by 7:00 p.m.

THURSDAY, OCTOBER 27:  MERCURY AT SUPERIOR CONJUNCTION
We bandy about the term "conjunction" almost as often as we plague the world with the term "light year."    Of course, we make matters worse by dividing conjunction into both "inferior conjunction" and "superior conjunction."   Today, we decided to discuss these conjunctions in greater detail.  A planet is at "superior" conjunction when it is on the far side of the Sun relative to Earth.   Both inferior planets (Mercury and Venus) and superior planets (those farther away from the Sun than Earth) can be in superior conjunction.    Refer to the image below.

A planet is at "inferior" conjunction when it is between Earth and the Sun.  Only the two inferior planets can ever be at inferior conjunction. 

​A solar system configuration diagram.    A planet is passing through "superior conjunction" when it passes on the other side of the Sun relative to Earth.    A planet passes through "inferior conjunction" when it moves between Earth and the Sun.  While both inferior and superior planets can be in "superior conjunction, only inferior planets can be in inferior conjunction.  Image by Planetfacts.org

FRIDAY, OCTOBER 28:    MOON 1.4 DEGREES NNE OF JUPITER
The moon and Jupiter will be low in the eastern pre-dawn sky tonight.    The waning crescent moon will only be 4% illuminated and will rise by 5:00 a.m, which is slightly more than two hours before sunrise and an hour and a half before the onset of civil twilight.    If you haven't yet had the chance to find Jupiter after its return, this morning you'll have no trouble finding it as it will appear so close to the crescent moon. 

SATURDAY, OCTOBER 29:  MARS AT PERIHELION
Planetary orbits are not perfect circles. Instead, they are ellipses, or more like ovals. Consequently, a planet's distance from the Sun changes constantly as it revolves around it.   The minimum distance separating a planet and the Sun is called "perihelion." The greatest distance is known as "aphelion."       Today, Mars reaches perihelion and will be 114.9 million miles from the Sun.      When at aphelion, Mars will be about 154 million miles from the Sun.    This 40 million mile difference between perihelion and aphelion is far greater than Earth, the heliocentric distance of which veers from a perihelion value of 91.5 million miles to an aphelion value of 94.5 million miles.    Mars' distance varies so much more because its orbit is more elongated than Earth's. 

SATURDAY, OCTOBER 29:   VENUS 3.0^o S of SATURN (GOLD EVENT!!!)
The brightest naked eye planet (Venus) appears close to the dimmest naked eye planet (Saturn).    Distinguishing between these two worlds therefore won't prove difficult, as Venus will be 69 times brighter than Saturn.  (Refer to "From the Catacombs of Infinite Knowledge" if you want to know how we arrived at this outrageous value.)   Both planets will set by 7:00 p.m.

PLANET WATCH
MERCURY  (Host constellation:  Virgo)  .     Mercury is now gone! 

VENUS   (Host constellation: Libra/Scorpius)  PICK PLANET!
Venus gets the crown again because it is so bright, even if it doesn't remain in the western evening sky for long.      Venus brightens only slightly throughout the month, but, as always, is brighter than any night sky star.  VERDICT:  It will be easy to find Venus provided you venture outside in the early evening.  Look to the west and see the brilliant Venus.   Best day to look this week will be Saturday, October 29, when Venus will appear close to Saturn.

MARS (Host constellation:   Sagittarius)
Though it is not as bright as Venus, Mars is easy to find in the western evening sky.  Interesting note:  Mars will be setting around 9:30 p.m. each night throughout the month.  This equivalence occurs because of the way Earth and Mars are moving relative to each other at the moment.   VERDICT:   Seek out the red world in the evening this month.    It will remain above the horizon for a couple hours after sunset.

JUPITER (Host constellation:  Virgo)
After a month-long hiatus, Jupiter retrurns to the early morning eastern sky.   Consider this to be a perfect "changing of the guard."  As Mercury vanishes in the pre-dawn twilight, Jupiter emerges.     Though not as bright as Venus, Jupiter shines brighter than any night sky star. VERDICT:  If you've dearly missed Jupiter, just venture outside to find it starting mid-month.  Jupiter will brighten throughout the rest of 2016 as it rises earlier each morning.   

SATURN  (Host constellation:  Libra)
We'll lose Saturn next month.      In October, Saturn lurks low in the southwestern evening sky.   It is still a moderately bright planet and will be easy to find, at least early in October.   VERDICT:   Seek out Saturn at month's beginning just after dark.     Still a beautiful world, particularly through a telescope.   Although we lose Saturn in November, we'll regain it in late December.  Best day to find Saturn will be October 29th,  when it will appear close to Venus in the southwestern early evening sky. 

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FROM THE CATACOMBS OF INFINITE KNOWLEDGE
"Calculating the brightness difference between two celestial objects"

Astronomers use the magnitude scale to measure the brightness of any given celestial object.  Devised by Hipparchus of Nicaea (190-120 BCE), this system assigns a number to a given object based on its brightness.    The brighter the object, the lower the number, which seems a bit counterintuitive.        For instance, a star of magnitude 1.0 is about 2.5 times brighter than a star of magnitude 2.    A star of magnitude 1.0 is 2.5 x 2.5 = 6.25 times brighter than a star of magnitude 3.0.        Calculating the brightness difference between stars of whole number magnitude is relatively straightforward.    For instance,  on Wednesday,  Oct 26,  Venus will be at magnitude -4.0 and Antares is at magnitude 1.0.     Venus is therefore 5.0 magnitudes brighter than Antares, or 100 times brighter!
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REMEMBER THAT DIMLY LIT WALL ETCHING?
The actual factor between magnitudes is closer to 2.512, which is a fifth root of 100.
Therefore, a star -or planet- that is five magnitudes brighter than another star or planet will be precisely 100 times brighter!
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However, how would one calculate the brightness difference between celestial objects if the magnitudes are not neat whole numbers?  For instance, on Saturday, Oct 29,  Venus will appear close to Saturn.  Venus will still be at magnitude -4.0. but Saturn's magnitude is 0.6.        What is the brightness difference?

We use the following formula

​

and we'll work through the Venus-Saturn example.

Venus' magnitude is -4.0.
Saturn's magnitude is 0.6

The absolute difference between these two values is 4.6.

We raise 2.5118…(the fifth root of 100) to 4.6 and yield 69.1 or 69.    Venus will be 69 times brighter than Saturn during their appulse on Saturday, October 29.
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