Eclipses, Lunar Phases and Supermoons

At a Glance

• Solar eclipse At new moon; Moon between Sun and Earth
• Lunar eclipse At full moon; Earth between Sun and Moon
• Moon’s orbit Tilted about 5 degrees to the Earth’s orbit
• Phase cycle About 29.5 days
• Supermoon Full or new moon at perigee
• Exam link UPSC Prelims, Geography: Earth, Latitudes, Longitudes and Motions

Where Does It Fit?

This is the sixth and last topic of the category on the Earth, latitudes, longitudes and motions. The previous topic covered solstices, equinoxes and day length, and this one covers the Moon and its alignment with the Earth and the Sun. The next category begins with the interior of the Earth. Previous topic: Solstices, Equinoxes and Variation in Day Length.

How Does the Moon Move?

In short: the Moon orbits the Earth once in about a month, and it always shows the same face.

The Moon is the only natural satellite of the Earth, at an average distance of about 384,400 km. It revolves around the Earth in about 27.3 days, and it rotates on its axis in the same period, so the same face of the Moon always points to the Earth. The cycle of phases, from one new moon to the next, takes about 29.5 days, which is longer because the Earth also moves around the Sun during the month. The Moon’s orbit is elliptical, so its distance changes: the nearest point is called perigee, about 3.6 lakh km away, and the farthest point is apogee, about 4.06 lakh km away. The orbit of the Moon is inclined at about 5 degrees to the orbital plane of the Earth around the Sun, which is why eclipses do not occur every month.

What Are the Phases of the Moon?

The Moon shines by reflected sunlight, and half of it is always lit. As it moves around the Earth, the angle between the Sun, Earth and Moon changes, so we see different portions of the lit half. This changing appearance is called the lunar phases, or the phases of the Moon.

The period from new moon to full moon is the bright fortnight (Shukla Paksha) and from full moon to new moon is the dark fortnight (Krishna Paksha) in the Indian lunar calendar.

How Does a Solar Eclipse Happen?

A solar eclipse occurs at the new moon, when the Moon passes between the Sun and the Earth and its shadow falls on a part of the Earth. The region in the dark central shadow (umbra) sees a total eclipse, while the region in the lighter outer shadow (penumbra) sees a partial eclipse. Types are:

• Total: the Moon completely covers the Sun, and the corona is visible. Totality at any place lasts a few minutes, at most about 7.5 minutes.
• Partial: only part of the Sun is covered.
• Annular: the Moon is near apogee and appears slightly smaller than the Sun, leaving a bright ring of light.
• Hybrid: the eclipse changes between annular and total along its path.

A total solar eclipse is seen only along a narrow track. The total solar eclipse of 12 August 2026 crossed Greenland, Iceland and Spain, and the next, on 2 August 2027, will cross North Africa and West Asia.

How Does a Lunar Eclipse Happen?

A lunar eclipse occurs at full moon, when the Earth lies between the Sun and the Moon and the Moon enters the Earth’s shadow. It can be total, partial or penumbral. Unlike a solar eclipse, it can be seen from the whole night side of the Earth, and it can last for hours; totality can last up to about 1 hour 47 minutes. During a total lunar eclipse the Moon often turns coppery red, because the Earth’s atmosphere bends red light into the shadow; this is popularly called a blood moon.

Why Is There Not an Eclipse Every Month?

If the Moon orbited in the same plane as the Earth, there would be a solar eclipse every new moon and a lunar eclipse every full moon. But the Moon’s orbit is tilted by about 5 degrees, so most months the Moon passes above or below the line joining the Sun and the Earth. Eclipses happen only when a new or full moon occurs near one of the two points where the Moon’s orbit crosses the Earth’s orbital plane, known as the nodes. Every year there are at least two and at most five solar eclipses, and the total number of eclipses of both kinds is usually four to seven.

What Are a Supermoon, a Blue Moon and a Blood Moon?

A supermoon is a full moon or a new moon that occurs when the Moon is at or near perigee. A supermoon full moon looks up to about 14 per cent larger and about 30 per cent brighter than a full moon at apogee. Supermoon is a popular term from astrology that was adopted widely by media, not a formal term of astronomy, although the underlying perigee-syzygy alignment is real.

• Blue moon: the second full moon in a calendar month; it does not mean the Moon looks blue.
• Blood moon: the reddish Moon during a total lunar eclipse.
• Spring tides: tides are highest at new moon and full moon, and perigee adds to them, as explained in Tides and Their Types.

Why Is This Topic Important?

• Eclipses Frequent in Prelims and current affairs
• Supermoon Common news term
• Phases Links to tides

What Came Next?

The next category begins with the interior of the Earth. The first topic is Sources of Information about the Interior of the Earth.

Key People and Terms

• Umbra Dark central shadow
• Penumbra Lighter outer shadow
• Perigee Moon nearest to the Earth
• Apogee Moon farthest from the Earth
• Annular eclipse Ring of light around the Moon
• Syzygy Sun, Earth and Moon in a line

Exam Corner

Mains Angle

Questions ask why eclipses are rare. These points add depth.

• Alignment Sun, Moon and Earth in a line
• Tilt Moon’s orbit inclined by 5 degrees
• Visibility Solar narrow, lunar wide