Positional Astronomy Flashcards
(24 cards)
What is 1° in arc minutes?
1° is 60’ i.e. 60 arc minutes or minutes of arc
What is 1’ in arc seconds?
1’ is 60’’ i.e. 60 arc seconds or seconds of arc
What is the arc length?
The arc of length S is the section of the circumference of a circle of radius r that subtends the angle θ.
What is the formula to calculate the arc length S of a circle radius r?
θ = S/r
Hence 360° =2π radians, since the circumference is 2πr.
What is latitude?
Circles on the surface of the Earth that are parallel to the equator
Latitude ranges from . . .
Latitude ranges from +90° to -90°, i.e. from North to South
What is considered the “zero point” of the latitude scale?
The equator is at 0° Latitude.
What is a great circle?
A great circle is a circle that has the same radius as the sphere on which it is drawn, the equator is a great circle.
What are small circles?
Apart from the equator, parallels of latitude don’t have the same radius as the sphere; such circles are termed small circles.
How does the longitude scale differ from the latitude scale?
All the longitude curves are great circles (to be precise semi-great circles)
What are meridians of longitude?
Curves of constant longitude are termed meridians of longitude, these are not parallel.
What is the “zero” point on the longitude scale?
The Greenwich meridian is an arbitrary zero.
How are longitude measurements made?
All longitude measurements are made with respect to the prime meridian:
1) >0 for East (0→180°)
2) <0 for West (0→-180°)
Define the Nautical mile
Nautical mile, S is that the fraction of the Earth is circumference subtending 1 minute of arc at the centre, as measured at the equator.
On what is spherical trigonometry based on?
Spherical trigonometry is based exclusively on great circles, that is, the lines with the minimum possible curvature on the surface of a sphere. The great circle distance between two points on a sphere is the shortest distance between those two points.
What is a spherical triangle?
A 3-sided figure made from intersections of great circles
Where do the great circles within a spherical triangle cut the equator?
90°
Angles in spherical triangles add up to …
> 180°
Why must every great circle arc intersect the equator at 90°?
It’s perfectly symmetric
Does the Earth rotate anti-clockwise or clockwise when viewed from above the North Pole?
Anti-clockwise. This means that the celestial sphere rotates clockwise when viewed from above the North Celestial Pole
The celestial sphere rotates clockwise when viewed from above the North Celestial Pole. What does this mean for when the celestial sphere is viewed from the Earth’s surface?
That it rotates anticlockwise
Define Diurnal motion
Diurnal motion refers to the apparent daily movement of celestial objects across the sky as a result of the earth’s rotation.
What is declination?
Declination is the equivalent of latitude on the celestial sphere, it is given by the symbol δ.
What is the Hour Angle
