Hemispheres

Think of an orange cut in half.

Each piece is half a ball. Hemi means half. So a hemisphere is half of a sphere, or half a ball.

The word hemisphere means half of a sphere. If you could slice Earth into two equal halves, each half would be a hemisphere.

National Geographic Education explains that you could do this in endless ways. Think of an orange: you could cut it in half in any direction. But geographers usually talk about four hemispheres: Northern, Southern, Eastern and Western. Two imaginary lines on the globe make the cuts.

A hemisphere is half of a sphere. National Geographic Education describes it as one of the two equal halves made when a circle is drawn around Earth's center; this can be done in infinitely many ways, since any great circle splits a sphere evenly.

In practice, four hemispheres are generally recognized. The Northern and Southern Hemispheres are separated by the Equator, the line of 0 degrees latitude. The Eastern and Western Hemispheres are separated along meridians, lines of longitude. The word is used for other halves too: National Geographic points out that the brain is also divided into left and right hemispheres.

The Equator goes around Earth's middle.

Everything above it is the Northern Hemisphere. Everything below it is the Southern Hemisphere.

The Equator divides Earth into the Northern Hemisphere and the Southern Hemisphere. NOAA's National Ocean Service says the Equator divides Earth equally into these two halves.

National Geographic lists what is in each. The Northern Hemisphere has North America, the northern part of South America, Europe, the northern two-thirds of Africa and most of Asia. The Southern Hemisphere has most of South America, one-third of Africa, Australia, Antarctica and some Asian islands.

The Equator, at 0 degrees latitude, divides Earth equally into the Northern and Southern Hemispheres. The two halves are equal in area but not in what they hold. National Geographic Education lists North America, the northern part of South America, Europe, the northern two-thirds of Africa and most of Asia in the Northern Hemisphere, and most of South America, one-third of Africa, Australia, Antarctica and some Asian islands in the Southern.

Every latitude carries an N or an S to say which half it is in: 30° N and 30° S are the same distance from the Equator, in different hemispheres.

Another line goes from the North Pole to the South Pole.

It cuts the Earth into an east half and a west half. The line goes through a place called Greenwich, in London.

To split Earth into east and west, geographers use lines of longitude. The prime meridian, at 0 degrees, runs through Greenwich in London. On the other side of the world is the line of 180 degrees, near the International Date Line. Together they divide Earth into the Eastern Hemisphere and the Western Hemisphere.

National Geographic notes that many geographers draw the line a little differently, at 20 degrees west and 160 degrees east, so that Africa and Europe are not split in two.

The prime meridian (0°) and the 180° meridian, the basis of the International Date Line, together form a great circle dividing the Eastern and Western Hemispheres. Longitudes east of Greenwich are measured in degrees east and those to the west in degrees west, so the letter E or W in a longitude names the hemisphere.

The east-west split is a convention, and not everyone uses the same one. National Geographic Education notes that many geographers instead use 20° W and 160° E, so that Africa and Europe are not cut in two. It also notes the older terms "Old World" for the Eastern Hemisphere and "New World" for the Western, which became significant after European nations began colonizing the Americas.

Every place is in two halves at once.

Sydney, in Australia, is in the south half and the east half. New York is in the north half and the west half.

Because there are two ways of cutting Earth in half, every place is in two hemispheres at once: one northern or southern, and one eastern or western.

Using city positions from Natural Earth, a free public map dataset: New York City is in the Northern and Western Hemispheres. Sydney, Australia, is in the Southern and Eastern Hemispheres. Nairobi, Kenya, is just south of the Equator, so it is in the Southern and Eastern Hemispheres. London is so close to the prime meridian that it sits in both the Eastern and Western Hemispheres, but its center is just west.

Every point on Earth (apart from those on the dividing lines) belongs to one north-south hemisphere and one east-west hemisphere, and its coordinates say which: the N or S of its latitude and the E or W of its longitude.

Natural Earth's city positions give some examples. New York City (40.75° N, 73.98° W) is Northern and Western. Sydney (33.92° S, 151.18° E) is Southern and Eastern. Nairobi (1.28° S, 36.81° E) lies just inside the Southern Hemisphere. Greater London straddles the prime meridian, so part of the city is in each of the Eastern and Western Hemispheres, though the point Natural Earth gives for its center (0.12° W) is just west. (Wikidata places Greenwich, home of the prime meridian, in south-east London.)

When it is summer in the north, it is winter in the south.

Some children have summer in July. Others have summer in December!

The Northern and Southern Hemispheres have opposite seasons. NASA explains that Earth's axis is tilted. When the North Pole tilts toward the Sun, the Northern Hemisphere gets more sunlight and has summer, while the Southern Hemisphere has winter. Half a year later, it is the other way around.

National Geographic says summer in the Northern Hemisphere runs from June through September, while in the Southern Hemisphere summer begins in December and ends in March.

Earth's axis is tilted 23.4 degrees to the plane of its orbit, according to NASA. For part of the year the Northern Hemisphere leans toward the Sun and receives more direct sunlight, giving it summer while the Southern Hemisphere has winter; the situation reverses six months later. NASA's Space Place stresses that this, not Earth's distance from the Sun, causes the seasons.

National Geographic gives the result in months: Northern Hemisphere summer from June through September, and Southern Hemisphere summer from December to March. Which north-south hemisphere a place is in sets the timing of its seasons; the east-west split makes no difference.