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PH1600: Introductory Astronomy
Lecture 5: Magnitudes and Calendars
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PH1600: Introductory Astronomy
Lecture 5: Magnitudes and Calenders
Next time: The Inner Solar System
School: Michigan Technological University
Professor: Robert Nemiroff
Online Course WebCT pages:
http://courses.mtu.edu/
This class can be taken online ONLY, class
attendance is not required!
You are responsible for…
Lecture
material
Listed
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wikipedia entries
But not higher math
APODs
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posted during the semester
APOD review every week during lecture
Completing
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the Homework-Quizzes
Homework 1 & 2 quizzes already due
Homework 3 quiz is due soon
See WebCT at http://courses.mtu.edu/
Wikipedia entries:
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Apparent magnitude
Absolute magnitude
Diurnal motion
Celestial pole
North star
Right ascension
Declination
Equinox
Solstice
Season
Calendar
Magnitudes
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Apparent magnitude
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How bright something appears
Absolute magnitude
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How bright something really is
Apparent magnitudes: m
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Higher magnitude means dimmer
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Counterintuitive, outdated, but
standard!
mag 5 star dimmer than mag 0 star
Scale is logarithmic
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5 magnitudes means a factor of 100
mag 0 star 100 times brighter than
mag 5 star
Relative magnitude formula
bA = 2.512(mB-mA) bB
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bA is the apparent brightness of star A
bB is the apparent brightness of star B
mA is the apparent magnitude of star A
mB is the apparent magnitude of star B
Relative magnitude examples
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Example: Star B appears one
magnitude fainter than star A. How
many times fainter is this?
Solution:bA/bB=2.512(mB–mA)
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bA/bB = 2.512(1) = 2.512 times fainter
Relative magnitude examples
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Example: Star B appears five
magnitudes fainter than star A.
How many times fainter is this?
Solution:bA/bB=2.512(mB–mA)
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bA/bB = 2.512(5) = 100 times fainter
Apparent magnitude table
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App. Mag.
Celestial object
−26.8
Sun
−12.6
full Moon
−9.5 Maximum brightness of an Iridium Flare
−4.4 Maximum brightness of Venus
−3.9 Faintest objects observable during the day with naked eye
−2.8 Maximum brightness of Mars
−1.5 Brightest star (except for the sun) at visible wavelengths: Sirius
−0.7 Second brightest star: Canopus
0
The zero point by definition: This used to be Vega
3
Faintest stars visible in an urban neighborhood
5.5
Maximum brightness of Uranus
6
Faintest stars observable with naked eye
7.7
Maximum brightness of Neptune
12.6 Brightest quasar
27
Faintest objects observable in visible light with 8m ground-based telescopes
30
Faintest objects observable in visible light with Hubble Space Telescope
http://en.wikipedia.org/wiki/Apparent_magnitude
Sirius: The Brightest Star in the Night
Credit & Copyright: Juan Carlos Casado
APOD: 2000 June 11
The Brightest Star Yet Known
Credit: D. F. Figer (UCLA) et al., NICMOS, HST, NASA
APOD: 1997 October 8,
APM 08279+5255: The Brightest Object Yet Known
Credit: M.J. Irwin (RGO) et al., Isaac Newton 2.5-m Telescope
APOD: 1998 August 18
Across the Universe
Credit: NASA Swift Team, Stefan Immler (GSFC) et al.
APOD: 2008 March 28
Rising and Setting Stars
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Earth’s rotation makes sky appear
to rotate one full turn each day
Celestial Sphere
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The sky: everything appears attached
to a distant rotating sphere
In Earth’s North: stars appear to
circle the North Celestial Pole
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Nearly occupied by the North Star:
Polaris
http://commons.wikimedia.org/wiki/Image:Celestial_Sphere.gif
Celestial Sphere Diagram
Northern Lights
Credit & Copyright: Philippe Moussette (Obs. Mont Cosmos)
APOD: 2004 July 30
Star Trails Above Mauna Kea
Credit & Copyright: Peter Michaud (Gemini Observatory), AURA, NSF
APOD: 2005 December 20
11 Hour Star Trails
Credit & Copyright: Josch Hambsch
APOD: 2006 September 15
Warped Sky: Star Trails Panorama
Credit & Copyright: Peter Ward
APOD: 2007 June 13
Rising and Setting Stars
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Zenith: The point on the Celestial
Sphere directly over the observer’s
head
Horizon: 90 degrees from your
zenith in all directions
Planets, the Sun, and the Moon rise
and set too just like the stars
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Earth’s rotation faster than their orbits
A Setting Sun Trail
Credit & Copyright: Anglo-Australian Observatory, Photograph by David Malin
APOD: 2002 July 29
Celestial Coordinates & Sky Labels
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Celestial Equator
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Right Ascension & Declination
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The circle midway between the north &
south celestial poles
Sky projection of Earth’s Equator
Like celestial latitude and longitude
Ecliptic
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The plane that the planes orbit the Sun
http://en.wikipedia.org/wiki/Right_Ascension
Reasons for Seasons
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NOT caused by Earth’s nearness to
the Sun
Caused by the tilt of the Earth
23.5 degree tilt points in the same
direction all year long
http://en.wikipedia.org/wiki/Image:North_season.jpg
Seasons of Northern Earth
Key Seasonal Dates
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Equinoxes: Equal Day and Night
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Vernal Equinox (March 22)
Autumnal Equinox (September 22)
Solstices: Maximum Day or Night
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Winter Solstice (North: December 22)
Summer Solstice (North: June 22)
Sunrise by Season
Credit & Copyright: Anthony Ayiomamitis
APOD: 2005 December 21
Sunrise Solstice at Stonehenge
Credit & Copyright: Pete Strasser (Tucson, Arizona, USA)
APOD: 2006 June 21
Manhattanhenge: A New York Sunset
Credit & Copyright: Neil deGrasse Tyson (AMNH)
APOD: 2007 July 13
Equinox + 1
Credit & Copyright: Joe Orman
APOD: 2004 March 20
Analemma over Ukraine
APOD: 2007 June 17
Calendar: Day Names
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Sunday = Sun Day
Monday = Moon Day
Tuesday = Mars (Twis) Day
Wednesday = Mercury (Woden) Day
Thursday = Jupiter (Thor) Day
Friday = Venus (Frige) Day
Saturday = Saturn Day
Calendars
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One year = time for Earth to go
once around the Sun
Does NOT correspond exactly to any
integer amount of Earth spins
(about 365.25 days)
Forgetting this causes your
Calendar to drift 0.25 days a year
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This can add up!
Calendars
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Roman Calendar
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Originally lunar, but kept changing
Lost many days per year
had occasional leap months
Julian Calendar
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Introduced in 46 BC by Julius Ceasar
365 days each year
One leap day every four years
Occurs in years evenly divisible by 4
Calendars
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Gregorian Calendar
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Decreed by Pope Gregory XIII in 1582
Accounted for inexactness of the “0.25” in
365.25 days per year
Took away a leap day every 100 years except
every 400 years
 Leap years: 1600, 2000
 Not leap years: 1700, 1800, 1900, 2100
Leap Seconds
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Now added many years to account for erratic
spin of the Earth
11 Hour Star Trails
Credit & Copyright: Josch Hambsch
APOD: 2006 September 15