Observe the following diagram and state which of the following alphabets indicate the gibbous phase of the moon ?

Question

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Correct — option (3), 'D and F'. The first thing to settle is that this question cannot be answered by looking at how dark the discs are. In the booklet all eight moons are drawn the same way, right half light and left half dark, so a candidate who hunts for the disc that 'looks fullest' finds nothing to choose between them. The diagram is not a picture of what the phases look like from Earth; it is a map of where the Moon is, and the phase has to be deduced from position. The one piece of information the drawing does give is the direction of the sunlight. Every one of the eight discs is lit on its right-hand side, so the Sun lies off the right-hand edge of the figure and its rays cross the page travelling leftwards. Fix that, and the whole ring falls into order around the Earth at the centre. A sits on the right, between the Earth and the Sun, so its sunlit face is turned away from us: A is the new moon. E sits directly opposite on the left, with the Earth between it and the Sun, so its whole sunlit face is turned towards us: E is the full moon. C at the top and G at the bottom stand at right angles to the Earth-Sun line, where exactly half of the near face is lit — these are the two quarter phases, the half moons. Gibbous means more than half of the visible disc lit but not yet the whole of it, and on this ring that is precisely the arc lying between a quarter position and the full moon. The two positions that satisfy it are the ones flanking E: D, between C and E, and F, between E and G. Hence option (3). The figure is symmetrical about the Earth-Sun line, and that symmetry is a free check on the answer, because every position in the upper half has a mirror twin below it in the same phase. B and H pair as the crescents, C and G as the quarters, D and F as the gibbous pair, and only A and E are left unpaired, because those two sit on the axis itself. Any answer that puts A and E together, or that mixes positions from two different pairs, has broken that symmetry.

Why the other options are wrong — (a) B and H are the crescent pair, not the gibbous pair. They flank A, which the direction of the shading identifies as the new moon, so they lie between new moon and quadrature and show less than half of the near face lit — the waxing crescent and the waning crescent. This option is the mirror image of the right answer, and it is chosen by a candidate who has correctly seen that the phases wanted are the two flanking an axis position but has attached them to the wrong end of the axis. The check that separates the two is the one thing the drawing states plainly: the lit halves face right, so the Sun is on the right and A is the moon caught between Earth and Sun. Had the Sun been drawn on the left, A would be full and E new, and B and H would indeed be the gibbous pair — which is exactly why the direction of the lighting is the first thing to read on any phase diagram.  (b) C and G are the quarter phases, and the quarter is the boundary of the gibbous range rather than a member of it. Both stand at right angles to the Earth-Sun line, so an observer on Earth sees exactly half of the near face lit: these are the first quarter and the last quarter, what is loosely called the half moon. Gibbous begins where the quarter ends — strictly more than half lit, strictly less than full. The option catches two kinds of candidate: the one who remembers the definition as 'between half and full' and then picks the half itself, and the one who assumes that the top and bottom of a circular diagram must be the significant positions because they are the easiest to see. Note also that C and G are as symmetrical a pair as D and F, so symmetry alone cannot decide between them; only the definition of the phase can.  (d) E and A are the two positions on the Earth-Sun axis itself, and they are the only two phases in the whole cycle that are neither crescent, quarter nor gibbous. A lies between the Earth and the Sun, giving the new moon, whose lit face is turned entirely away from us; E lies on the far side of the Earth from the Sun, giving the full moon, whose lit face is turned entirely towards us. These are the syzygies, the alignments at which eclipses become possible, though not at every new or full moon, because the Moon's orbit is tilted to the plane of the Earth's orbit. Choosing this option generally means that gibbous has been read as a synonym for a fully lit moon. It is not: the word comes from the Latin for a hump, and it describes a disc that bulges beyond a half but stops short of a circle, so a full moon is by definition not gibbous.

Concept: The Moon produces no light of its own and the Sun always lights exactly one half of it. What changes through the month is not how much of the Moon is lit but how much of the lit half an observer on Earth can see, and that depends purely on the angle between the Sun, the Earth and the Moon as the Moon travels round its orbit. When the Moon is between the Earth and the Sun, its lit hemisphere faces away from us and we see the new moon. As it moves on, a sliver of the lit side swings into view and grows: waxing crescent, then first quarter when the Sun-Earth-Moon angle reaches ninety degrees and exactly half the visible disc is lit, then waxing gibbous as the lit portion passes a half, then full moon when the Earth lies between the Sun and the Moon. The sequence then reverses through waning gibbous, last quarter and waning crescent back to new. Gibbous therefore occupies the two arcs on either side of full, and crescent the two arcs on either side of new, with the quarters as the boundaries between them. Note what the phases are not: they have nothing to do with the Earth's shadow falling on the Moon, which is a lunar eclipse and a separate, occasional event. A phase diagram of this kind is drawn from above the orbit, so it shows positions rather than appearances, and the reader has to convert one into the other.

State PCS 2023 Q21 — Geography PYQ with Answer

State PCS 2023 Q21 — Geography PYQ with Answer