The Moon Lines Up Every Month yet Eclipses Are Rare

Because the Moon's path is tilted. At most new Moons it passes a little above or below the direct line from the Sun to the Earth, so no shadow falls where it would need to.

The line-up happens every month; the perfect line-up does not. This page covers everything in the CBSE Class 8 Science chapter's first part: the phases of the Moon, predicting where a phase appears, why phases occur, and why eclipses are uncommon.

The phases are the changing shapes of the lit part of the Moon as seen from Earth, and they repeat in a fixed cycle of about 29 and a half days, called a lunar month.

In order from new Moon:

- New Moon (amavasya) — no lit part visible at all
- Waxing crescent — a thin curved sliver
- First quarter (half Moon) — exactly half lit
- Waxing gibbous — more than half, less than full
- Full Moon (purnima) — the whole disc lit
- Waning gibbous — shrinking from full
- Last quarter (half Moon) — half lit again, the other half this time
- Waning crescent — a thin sliver again
- back to new Moon

The two halves of the cycle have names:

- Waxing — the lit part is growing, from new Moon to full Moon. This is the bright fortnight, shukla paksha.
- Waning — the lit part is shrinking, from full Moon to new Moon. This is the dark fortnight, krishna paksha.

The Indian calendar is built on exactly this, which is why festivals fall on named days of a fortnight.

The way to tell waxing from waning at a glance is which side is lit, and it is worth fixing. In the northern hemisphere a waxing Moon is lit on its right side, and a waning Moon on its left — so two crescents of identical shape can belong to opposite halves of the month.

It depends on the phase, because each one sits at a different angle from the Sun as seen from Earth.

- Full Moon — opposite the Sun, so it rises around sunset, is high overhead near midnight, and sets around sunrise. Visible all night.
- New Moon — in nearly the same direction as the Sun, so it is in the sky by day and lost in the glare. Not visible at all.
- Waxing crescent — a little east of the Sun, so it is seen low in the western sky just after sunset, and sets soon after.
- First quarter — rises around midday and sets around midnight, so it is high in the sky in the early evening.
- Last quarter — rises around midnight, so it is high in the early morning sky and visible before dawn.
- Waning crescent — seen low in the eastern sky just before sunrise.

So anyone who has looked for a thin crescent knows it must be done shortly after sunset, facing west — wait two hours and it has set.

Recording observations makes the pattern obvious. Sketch the Moon's shape and note the time and direction each evening for a fortnight, and you will find it rising roughly 50 minutes later each day while its lit part grows.

That daily delay is the part students find surprising, and it explains the rest. The Moon moves along its orbit while the Earth turns, so it does not return to the same place at the same clock time — which is why a phase is tied to a time of night as much as to a shape.

Because the Moon does not give out its own light — it shines by reflected sunlight — and we see only the part that is both lit by the Sun and facing the Earth.

The Sun always lights one half of the Moon, exactly as it lights one half of the Earth. What changes through the month is how much of that lit half we can see, and that depends on the relative positions of the Sun, Earth and Moon as the Moon orbits us.

- New Moon — the Moon lies roughly between the Sun and the Earth, so its lit half faces away from us and we see none of it.
- Full Moon — the Earth lies between the Sun and the Moon, so the lit half faces us fully.
- Quarters — the Moon is off to one side, so we see half of the lit half, which appears as a half disc.

A classroom model makes it immediate: hold a ball beside a lamp and walk around it, and the lit portion you can see changes through exactly these shapes.

The misconception worth clearing is that phases are caused by the Earth's shadow falling on the Moon. They are not. The Earth's shadow is involved only in a lunar eclipse, which is a separate and much rarer event. Phases happen because of viewing angle alone — the Moon's dark part is simply the half that the Sun is not lighting.

Because the Moon's orbit is tilted relative to the Earth's path around the Sun, so the three bodies rarely fall in an exact straight line.

First, what an eclipse needs:

- A solar eclipse happens at new Moon, when the Moon comes between the Sun and the Earth and its shadow falls on part of the Earth.
- A lunar eclipse happens at full Moon, when the Earth comes between the Sun and the Moon and the Earth's shadow falls on the Moon.

So the positions are right every single month — new Moon and full Moon occur once each in every lunar month.

What is usually wrong is the alignment in height. Because the Moon's orbit is tilted by a few degrees, at most new Moons the Moon passes above or below the Sun-Earth line, so its shadow misses the Earth entirely. At most full Moons it passes above or below the Earth's shadow, so nothing darkens it.

Only when a new or full Moon happens to occur near the points where the two orbital planes cross do the three line up closely enough, and that is why eclipses come only a few times a year rather than twice a month.

A solar eclipse is rarer still at any one place, and the reason is worth stating. The Moon's shadow on Earth is small, so only a narrow strip of the surface sees the Sun fully covered — while a lunar eclipse, once it happens, is visible from the whole night side of the Earth at once.