Lecture 2 - Fundamentals of Geodetic Astronomy Flashcards
*Where is geodetic astronomy concerned?
Astronomical latitude (phi), astronomic longitude (lambda), astronomic azimuth (alpha).
Astronomic measurements depend on Earth’s rotation, a precise time system and coordinate and time systems.
What is the apparent motion of a celestial object in astronomy?
The motion caused by the Earth makes the whole sky seem like it is actually rotation.
*What is the celestial sphere?
An imaginary sphere with the Earth at its center. The celestial bodies appear to be located on this sphere, and the sphere appears to rotate around the Earth’s extended axis once every 24 hours carrying the celestial bodies with it overhead and giving them their diurnal motions.
A spherical map of the sky that provides the basis for the coordinate systems used in celestial navigation and in specifying the positions and motions of celestial objects.
A practical tool for astronomy, to plot positions of objects in the sky when their distances are unknown or unimportant.
What are the typical measurements from the observation of a celestial object using a theodolite on the Earth?
Azimuth and Altitude.
How to define the position of a star on the celestial sphere?
To define the position of a star anywhere on the celestial sphere, a direction is needed.
Direction is most simply defined as a unit vector in polar (curvilinear) coordinates : r, theta, lambda. R = 1 (unit vector, as a distance not observed on an infinite celestial sphere). Thus, the vector is defined by only two angles.
All coordinate systems defined here are ‘dynamical’ in nature, though they have recently been replaced by ‘kinematical’ systems designed, in part, to mimic them.
What is the ‘apparent’ coordinate system?
The designation ‘apparent’ will signify that the origin of a celestial coordinate system is at the center of mass of the Earth (geocentric).
What is the ‘heliocentric’ coordinate system?
When ‘apparent’ is not used, the system will be heliocentric – the sun as the center.
The position of a star is different from two observers at different locations on the Earth. Explain.
A celestial coordinate system is defined by the location of its origin and the directions of the axes towards certain characteristic points on the celestial sphere whose positions are defined dynamically using celestial mechanics.
What does the prefix ‘instantaneous’, ‘true’, or ‘mean’ mean when one talks about celestial coordinate systems?
Celestial coordinate systems that are affected by all irregularities and at all frequencies of the rotation axis of the Earth (ie precession and nutation) will be designated as ‘instantaneous’ or ‘true’ and will be shown to be functions of time.
When a celestial system precesses but does not nutate, it will be called ‘mean’.
Is the Earth’s North pole a fixed point? If no, what are the phenomena that cause the irregularities of the rotation axis of the Earth?
No. The irregularities are precession-nutation, diurnal rotation, and polar motion.
What is diurnal rotation?
Around the celestial intermediate pole through the nominal Earth rotation rate and corrections showing variations of the Earth angular velocity.
What is polar motion?
The motion of the celestial intermediate pole with respect to the terrestrial crust.
*What is the Right Ascension System?
The most important celestial system in which star coordinates are published, also serves as the connection between terrestrial, celestial and orbital coordinate systems.
Primary reference plane: the celestial equator.
Secondary plane: the equinoctial colure (the hour circle passing through two polar points NCP and SCP and the vernal and autumnal equinoxes.
Star coordinates: right ascension, alpha, and the declination, delta.
Have a heliocentric origin.
The RA coordinates are time dependent.
*What is the Local Horizon System?
Primary reference plane: the celestial horizon
Secondary plane: the observer’s celestial meridian
Star coordinates: the position of a celestial body (CB) in a system peculiar to a topographically located observer (Altitude & Azimuth).
Altitude h: the angle between the celestial horizon and the point CB measured in the plane of the vertical circle (0-90).
Azimuth a: the angle between the observer’s celestial meridian and the vertical circle through CB (0-360, north to east, clockwise)
Having a terrestrial (topocentric) origin
*What is the hour angle system?
Primary reference plane: the celestial equator
Secondary plane: the hour circle containing the observer’s celestial meridian (the zenith)
Star coordinates: The hour angle t (the angle between the hour circle of CB and the observer’s celestial meridian/hour circle in the plane of the celestial equator: 0-24h, clockwise). The declination delta (the angle between the celestial equator and the point CB in the plane of the hour circle through the point CB:0-90)
Location and time dependent
