Which one of the following statements about ‘great circle’ is not correct?
- (a)Every great circle divides the Earth into equal halves.
- (b)Every great circle is a circumference of the Earth.
- (c)Great circles mark the longest travel routes between locations on the Earth’s surface.
- (d)Great circle is the largest circle that can be drawn around the Earth through two particular points.
Correct — C, Great circles mark the longest travel routes between locations on the Earth's surface. They mark the shortest. Any circle whose plane passes through the centre of the Earth cuts the globe into two equal halves, and the shorter arc of such a circle between two places is the least distance between them along the surface — which is why long-haul flights from Delhi to New York bend up over the Arctic instead of running straight across the map. The remaining three statements state the definition correctly.
- (a)Every great circle divides the Earth into equal halves. — True by definition — the plane of a great circle passes through the Earth's centre, so it must bisect the globe.
- (b)Every great circle is a circumference of the Earth. — True. A great circle has the same radius as the Earth itself, so its length equals the Earth's circumference, roughly 40,000 km.
- (d)Great circle is the largest circle that can be drawn around the Earth through two particular points. — True. Through any two points that are not diametrically opposite, exactly one great circle can be drawn, and no larger circle through them exists.
Cut a sphere with a plane through its centre and the edge of the cut is a great circle; cut it anywhere else and you get a small circle. On the Earth the Equator is a great circle and every meridian pairs with its antimeridian to make one, while the Tropic of Cancer and every other parallel is a small circle.
The whole item turns on one word. Students meet the phrase 'great circle route' first in aviation, where it always means the economical path, so 'longest' should ring wrong immediately. The reason a great-circle route looks curved on an atlas is the projection, not the path: a Mercator map straightens the constant-bearing rhumb line and bends the shorter great-circle arc.
- A great circle's plane passes through the Earth's centre; its length is the Earth's circumference, about 40,000 km.
- The Equator is the only parallel of latitude that is a great circle; all others are small circles.
- Any meridian together with its opposite meridian forms one great circle.
- The shorter arc of the great circle through two points is the shortest surface distance between them.
- On a Mercator chart the rhumb line is straight and the great-circle route appears curved.
- Reading 'largest circle' in one statement as 'longest route' in another.
- Calling every meridian a great circle when it is only half of one.
One false statement among four, or a list of lines asking which of them are great circles.
Which of the following statements with reference to lines of latitudes is/are correct? 1. The distance between two successive latitudes changes slightly from the equator to the poles. 2. If parallels of latitude are drawn at an interval of one degree, the total number of parallels thus drawn, including the equator, will be 179. Select the answer using the code given below:
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(c) Both 1 and 2
The other half of the graticule: CDS asked how the parallels of latitude behave and how many of them a one-degree interval produces, where this item asks what makes a circle great.
- practice — not a real PYQ
Which one of the following is a great circle?
- (a)the Tropic of Cancer
- (b)the Arctic Circle
- (c)the Equator
- (d)the parallel of 45° N
Answer(c) the Equator — it is the only parallel whose plane passes through the Earth's centre.
- practice — not a real PYQ
Aircraft on long intercontinental flights follow great-circle routes because such a route is
- (a)the shortest surface distance between two points
- (b)always a straight line on a Mercator map
- (c)free of headwinds
- (d)the path of constant compass bearing
Answer(a) the shortest surface distance between two points — the saving in distance is why the track bends towards the pole.