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:

Correct — C, Both 1 and 2. Take statement 2 first, because it settles most of the question by itself. If parallels are drawn at intervals of one degree, there are 89 of them between the equator and the North Pole — at 1 degree, 2 degrees and so on up to 89 — and 89 more in the southern hemisphere, and the equator itself makes one more. That is 89 plus 89 plus 1, or 179. The reason the count stops at 89 rather than 90 is that a pole is a point, not a circle: a parallel at 90 degrees would have zero length and is not a parallel at all. Any answer that rejects statement 2 is therefore wrong before statement 1 is even considered. Statement 1 holds too. The Earth is an oblate spheroid, flattened at the poles, so the curvature of a meridian is not uniform and one degree of latitude does not measure the same length everywhere. On the reference spheroid used for global positioning the arc of one degree of latitude is 110.574 kilometres at the equator, 110.852 kilometres at 30 degrees, 111.412 kilometres at 60 degrees and 111.694 kilometres at the pole. The whole spread is a little over one kilometre in about a hundred and eleven, which is exactly the 'slightly' the paper writes into the statement. On a perfect sphere the value would be a constant 111.2 kilometres, and the departure from that constant is a direct measure of the Earth's flattening.

• (a)1 only — Accepts the variation in the degree of latitude but rejects the count. The arithmetic of the count is not in doubt: 89 parallels in each hemisphere plus the equator gives 179.
• (b)2 only — The defensible near-miss. It follows the school-text simplification that the spacing of parallels is uniform, which is a good enough approximation for a classroom globe but not literally true of an oblate spheroid — and the paper's word 'slightly' points at the exact quantity the simplification drops.
• (d)Neither 1 nor 2 — Cannot be right under any reading, because statement 2 is a plain count that comes out at 179. Rejecting it would require a parallel at each pole, and a pole has no circumference.

Parallels of latitude are circles drawn parallel to the equator, numbered from 0 degrees at the equator to 90 degrees at each pole. They shrink as they go poleward — the equator is a great circle of about 40,075 kilometres, while the Arctic Circle is far shorter — and they end in points rather than circles at 90 degrees north and south. Meridians of longitude behave the other way round: all of them are half great circles of equal length, they converge on the poles, and their spacing is greatest at the equator and zero at the poles. That asymmetry is why counting parallels and counting meridians give different totals, and why the length of a degree of longitude changes enormously with latitude while the length of a degree of latitude changes only slightly.

This item rewards a student who separates two things that look alike. The distance between successive parallels is a north-south measurement along a meridian, and it changes only a little, because it depends on the curvature of the meridian. The distance between successive meridians is an east-west measurement along a parallel, and it changes enormously, from about 111 kilometres at the equator to nothing at the poles. Candidates who have learnt the second fact well often import it into the first and conclude that the spacing of parallels must vary a lot, or, going the other way, learn that a degree of latitude is 'about 111 kilometres everywhere' and conclude that it never varies at all. Both statements in this question are the careful version. One further point: the variation runs the opposite way to intuition. The degree of latitude is longest at the poles, where the Earth is flattest and the meridian least curved, and shortest at the equatorial bulge.

• One degree of latitude measures 110.574 km at the equator and 111.694 km at the pole on the WGS84 spheroid — longest at the poles, shortest at the equator.
• On a perfect sphere the value would be a constant 111.2 km; the departure from it measures the Earth's oblateness.
• Parallels at one-degree intervals number 89 in each hemisphere plus the equator, giving 179 in total, because the poles are points rather than circles.
• Meridians of longitude at one-degree intervals number 360, all of equal length, converging at the poles.
• One degree of longitude measures about 111.320 km at the equator, 96.486 km at 30 degrees, 55.800 km at 60 degrees and zero at the poles.

• Carrying the strong variation in the length of a degree of longitude across to the length of a degree of latitude, which varies only slightly.
• Counting a parallel at 90 degrees. The poles are points, so the count of one-degree parallels is 179 and not 181.
• Expecting the degree of latitude to be longest at the equator because the Earth is widest there. It is longest at the poles, where the meridian is least curved.

As a two- or three-statement item on the properties of the graticule, often mixing a counting claim with a claim about distances.

• practice — not a real PYQ

  If meridians of longitude are drawn at intervals of one degree, how many will there be in all?

  • (a)179
  • (b)180
  • (c)360
  • (d)361

  Answer(c) 360 — longitude runs from 0 to 180 degrees east and 0 to 180 degrees west, and the two 180-degree meridians are the same line, so the count comes to 360.

• practice — not a real PYQ

  The length of one degree of longitude is greatest at which one of the following?

  • (a)The equator
  • (b)The Tropic of Cancer
  • (c)The Arctic Circle
  • (d)The North Pole

  Answer(a) The equator — about 111.320 km there, falling to about 55.800 km at 60 degrees and to zero at the poles, where the meridians meet.