The Third Quarter Moon / Half Moon
The Third Quarter Moon is when the opposite half of the Moon is illuminated compared to the First Quarter. Which half you see lit up depends on where you are on Earth.
The Third or Last Quarter Moon is also called a Half Moon because we can see exactly 50% of the Moon's visible surface illuminated.
Sun Lights Up the Moon
The Moon does not radiate its own light, but the Moon's surface reflects the Sun’s rays.
Half of the Moon’s surface is always illuminated by direct sunlight, except during lunar eclipses when Earth casts its shadow on the Moon. Just how much of that light we can see from Earth varies every day, and we call this Moon phase.
Last Primary Moon Phase
In Western Culture, we divide the lunar month into four primary and four intermediate Moon phases.
Third Quarter Moon is the last primary phase when the Moon has reached the third, or last, quarter of its orbit around Earth, hence the name.
Technically, the primary Moon phases occur at a specific moment in time. The time between the primary phases is called intermediate Moon phases. These are the Waxing Crescent Moon, the Waxing Gibbous Moon, the Waning Gibbous Moon, and the Waning Crescent Moon.
Same Phase Looks Different
Moon phases are the same all over the world. The same percentage and area of the Moon are illuminated no matter where on Earth you are. However, the Moon is rotated in different ways depending on the time, the date, your location, and the Moon's position in the sky. Therefore, the illuminated part can appear on the left, the right, the top, or the bottom.
At Third Quarter in the northern regions of the world, the left half of the Moon is lit up, while the right half is illuminated in the southern regions. Near the equator, the lower part is bright after moonrise, and the upper part is bright before moonset.
On the day of Third Quarter, the Moon rises approximately in the middle of the night and sets in the middle of the day.
Affects the Tides
The ocean tides on Earth are mostly generated by the Moon’s gravitational pull. At First and Third Quarter, the Moon and Sun pull in different directions, producing the smallest difference between high and low tide, known as neaps or neap tide.
The largest tidal range is during around Full Moon and New Moon. During these Moon phases, the gravitational forces of the Moon and the Sun combine to pull the ocean’s water in the same direction. These tides are known as spring tides or king tides.
Third Quarter Moon in Calendars
The symbol for Third Quarter Moon in modern calendars is a circle split down the middle with the left side white and the right side black.
The other primary Moon phase symbols in calendars:
= New Moon = First Quarter = Full Moon
The Moon phase calendar symbols reflect the Moon's appearance in the Northern Hemisphere, which can be confusing for people in the Southern Hemisphere, where the opposite side is illuminated at The First and Third Quarter.
The Moon illustration on our Moon phase pages indicates more accurately, although not perfectly, which part of the Moon is lit up in more than 5000 locations worldwide.