train:lectures:positionalastro

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train:lectures:positionalastro [2024/03/18 12:47] – Roy Proutytrain:lectures:positionalastro [2024/03/18 16:10] (current) – Roy Prouty
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-The March Equinox is chosen to be the zero-point for the Equatorial Coordinate analogous to latitude -- the Right Ascension or RA. RA is measured eastward along the Celestial Equator starting at the March Equinox.+The March Equinox is chosen to be the zero-point for the Equatorial Coordinate analogous to latitude -- the Right Ascension or RA. RA($\alpha$) is measured eastward along the Celestial Equator starting at the March Equinox.
 ==Milankovitch Cycles== ==Milankovitch Cycles==
 The specific location of the CNP/CSP/CEq do change over time in cycles known as {{https://en.wikipedia.org/wiki/Milankovitch_cycles|Milankovitch Cycles}}. These cycles are symptomatic of the Precession/Nutation of the Earth's rotation axes, Orbital Eccentricity, and Orbital Obliquity. They're due to the gravitational effects of the masses in our Solar System being more in-line with the Ecliptic than our Celestial Equator! How fun!?   The specific location of the CNP/CSP/CEq do change over time in cycles known as {{https://en.wikipedia.org/wiki/Milankovitch_cycles|Milankovitch Cycles}}. These cycles are symptomatic of the Precession/Nutation of the Earth's rotation axes, Orbital Eccentricity, and Orbital Obliquity. They're due to the gravitational effects of the masses in our Solar System being more in-line with the Ecliptic than our Celestial Equator! How fun!?  
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-Declination is measured in degrees, minutes of arc and seconds of arc with $360^\circ$ if traversing any Great Circle containing the CNP and CSP. Right Ascension is measured in hours, minutes, and seconds with $24^h$ if traversing the full Celestial Equator. As we shall see, changes in RA correspond to different angular rotations of the Earth -- or different times of day! These units can be confusing. Minutes of arc (or arcminutes) are NOT the same as minutes of RA. Furthermore, the notation of RA & DEC can be confusing. They can be given a degree-decimals, or fully-qualified $(D_1^\circ~D_2'~D_3'', R_1^h, R_2^m, R_3^s), or some fun mixture of the two! +Declination is measured in degrees, minutes of arc and seconds of arc with $360^\circ$ if traversing any Great Circle containing the CNP and CSP. Right Ascension is measured in hours, minutes, and seconds with $24^h$ if traversing the full Celestial Equator. As we shall see, changes in $\alpha$ correspond to different angular rotations of the Earth -- or different times of day! These units can be confusing. Minutes of arc (or arcminutes) are NOT the same as minutes of RA. Furthermore, the notation of $\alpha$ & $\delta$ (RA & DEC) can be confusing. They can be given in their fully qualified notation $(D_1^\circ~D_2'~D_3'', R_1^h, R_2^m, R_3^s)$, as degree-decimals, or some fun mixture of the two! When looking at a very small Field of View, angular measures in $\alpha$ will frequently just be in terms of arcminutes or arcseconds. Pay careful attention to these notations and values. Be sure you understand the differences between them!
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-==== Reconciling Both Systems ====+==== Reconciling LCHS and Eq Systems ====
 With this Equatorial Coordinate System, an observer  With this Equatorial Coordinate System, an observer 
 [{{:train:lectures:screen_shot_2024-03-18_at_12.35.44_pm.png?400|The LHCS projected onto the Celestial Sphere centered on the fundamental plane of the  Equatorial Coordinate System.}}] [{{:train:lectures:screen_shot_2024-03-18_at_12.35.44_pm.png?400|The LHCS projected onto the Celestial Sphere centered on the fundamental plane of the  Equatorial Coordinate System.}}]
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 +For an observer at a positive latitude, $\lambda$, r ($\lambda=~39^\circ$ for MD):
 +  - the CNP will be located at $(ALT=\lambda^\circ, AZ=0^\circ)$
 +  - the CSP will never be above the local horizon (Why?)
 +  - the CEq will intersect the local meridian at $(ALT=90^\circ - \lambda, AZ=180^\circ)$
 +  - Zenith will see changing RA, depending on the local time, but the $\delta$ will always be $\lambda^\circ$
  
 +=====Stellarium Exercises=====
 +Using Stellarium, let’s all do the following together …
  
 +  - Set location to Baltimore. Note the (lat, lon)
 +      - Identify the Ecliptic
 +      - Set the date to your birthday and tell me your “Sun sign” – ugh
 +      - Explore Local Horizon Grid
 +          - Find the Pole Star and find its (alt, az) – what should it be?
 +          - Enable the Local Meridian
 +          - Play time forward and watch star paths
 +      - Explore Equatorial Grid
 +          - Enable CEq
 +          - Play time forward and watch star paths 
 +          - Set location to North Pole, South Pole, Equator, etc
 +      - Determine (alt, az) of a star in Orion, determine (RA, DEC)
 +      - Determine exact time (ET) of March Equinox
 +      - Back in Baltimore, identify the set of stars that never set
 +          - These stars comprise the Circumpolar Region of our Local Horizon
 +          - How does this region change with latitude?
 +      - With Local Meridian on over Equatorial Grid: Define Transits & Sidereal Time
 +          - Look up current Sidereal Time: https://www.localsiderealtime.com/ 
 +
 +----
 +
 +Written by Roy Prouty 20240318\\
 +Reviewed by