The word solstice is derived from the Latin sol (sun) and sistere (to stand still), because at the solstices, the Sun stands still in declination; that is, the seasonal movement of the Sun’s path (as seen from Earth) comes to a stop before reversing direction. A solstice is an astronomical event that occurs twice each year (in June and December) as the Sun reaches its highest or lowest excursion relative to the celestial equator on the celestial sphere. The seasons of the year are directly connected to both the solstices and the equinoxes. The term solstice can also be used in a broader sense, as the day when this occurs. The day of the solstice is either the longest day of the year (summer solstice) or the shortest day of the year (winter solstice) for any place outside of the tropics. Alternative terms, with no ambiguity as to which hemisphere is the context, are June solstice and December solstice, referring to the months of year in which they take place. At latitudes in the temperate zone, the summer solstice marks the day when the sun appears highest in the sky. However, in the tropics, the sun appears directly overhead (called the sub solar point) some days (or even months) before the solstice and again after the solstice, which means the sub solar point occurs twice each year.
An equinox is an astronomical event in which the plane of Earth’s equator passes through the center of the Sun, which occurs twice each year, around 20 March and 23 September. On an equinox, day and night are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction. To avoid this ambiguity, the word equilux is sometimes used to mean a day in which the durations of light and darkness are equal.
Day is usually defined as the period when sunlight reaches the ground in the absence of local obstacles. On the day of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, so night and day are about the same length. In reality, the day is longer than the night at an equinox. There are two reasons for this:
First, from the Earth, the Sun appears as a disc rather than a point of light, so when the centre of the Sun is below the horizon, its upper edge is visible. Sunrise, which begins daytime, occurs when the top of the Sun’s disk rises above the eastern horizon. At that instant, the disk’s centre is still below the horizon.
Second, Earth’s atmosphere refracts sunlight. As a result, an observer sees daylight before the top of the Sun’s disk rises above the horizon. Even when the upper limb of the Sun is 0.4 degrees below the horizon, its rays curve over the horizon to the ground.
In sunrise/sunset tables, the assumed semi diameter (apparent radius) of the Sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc. Their combination means that when the upper limb of the Sun is on the visible horizon, its centre is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. These effects make the day about 14 minutes longer than the night at the equator and longer still towards the poles. The real equality of day and night only happens in places far enough from the equator to have a seasonal difference in day length of at least 7 minutes, actually occurring a few days towards the winter side of each equinox. The times of sunset and sunrise vary with the observer’s location (longitude and latitude), so the dates when day and night are equal also depend upon the observer’s location. At the equinoxes, the rate of change for the length of daylight and night-time is the greatest. At the poles, the equinox marks the transition from 24 hours of nighttime to 24 hours of daylight (or vice versa).
3. International Date Line
The International Date Line (IDL) is an imaginary line of navigation on the surface of the Earth that runs from the north pole to the south pole and demarcates the change of one calendar day to the next. It passes through the middle of the Pacific Ocean, roughly following the 180° line of longitude but deviating to pass around some territories and island groups. The IDL is roughly based on the meridian of 180° longitude, roughly down the middle of the Pacific Ocean, and halfway around the world from the Greenwich meridian. In many places, the IDL follows the 180° meridian exactly. However, in other places, the IDL deviates east or west away from that meridian. These various deviations generally accommodate the political and/or economic affiliations of the affected areas.
Proceeding from north to south, the first deviation of the IDL from 180° is to pass to the east of Wrangel Island and the Chukchi Peninsula, the easternmost part of Russian Siberia. It then passes through the Bering Strait between the Diomede Islands at a distance of 1.5 kilo meters (0.93 mi) from each island. It then bends considerably west of 180°, passing west of St. Lawrence Island and St. Matthew Island. It crosses between the Aleutian Islands, belonging to the US—Attu Island being the westernmost—and the Commander Islands belonging to Russia. It then bends southeast again to return to 180°. Thus all of Russia is to the west of the IDL and all of the United States is to the east.
The IDL remains on the 180° meridian until passing the equator. Two US-owned uninhabited atolls, Howland Island and Baker Island, just north of the equator in the central Pacific Ocean (and ships at sea between 172.5°W and 180°) have the latest time on Earth of UTC-12 hours. The IDL circumscribes Kiribati by swinging far to the east, almost reaching the 150°W meridian. Kiribati’s easternmost islands, the southern Line Islands south of Hawaii, have the most advanced time on Earth, UTC+14 hours. South of Kiribati, the IDL returns westwards but remains east of 180°, passing between Samoa and American Samoa. In much of this area, the IDL follows the 165°W meridian. Accordingly, Samoa, Tokelau,Wallis and Futuna, Fiji, Tonga, Tuvalu and New Zealand’s Kermadec Islands and Chatham Islands are all west of the IDL and have the same date. American Samoa, the Cook Islands, Niue, and French Polynesia are east of the IDL and one day behind.The IDL then bends southwest to return to 180°. It follows that meridian until reaching Antarctica. Conventionally, the IDL is not drawn into Antarctica on most maps.
4. The Doldrums
The doldrums is a colloquial expression derived from historical maritime usage, which refers to those parts of the Atlantic Ocean and the Pacific Ocean affected by the Inter tropical Convergence Zone, a low-pressure area around the equator where the prevailing winds are calm. The doldrums are also noted for calm periods when the winds disappear altogether, trapping sail-powered boats for periods of days or weeks. The term appears to have arisen in the eighteenth century, when cross-equator sailing voyages became more common. Since this zone is the meeting place of two trade winds, it is also called inter tropical convergence zone. n maritime usage, the low pressure characteristics of the doldrums is caused by the expanding atmosphere due to heating at the equator, which makes the air rise and travel north and south high in the atmosphere, until it subsides again in the horse latitudes. Some of that air returns to the doldrums through the trade winds. This process can lead to light or variable winds and more severe weather, in the form of squalls, thunderstorms, and hurricanes. The doldrums are also noted for calm periods when the winds disappear altogether, trapping sail-powered boats for periods of days or weeks.
5. Horse latitudes
Horse latitudes or subtropical highs are subtropical latitudes between 30 and 38 degrees both north and south where Earth’s atmosphere is dominated by the subtropical high, an area of high pressure, which suppresses precipitation and cloud formation, and has variable winds mixed with calm winds. The horse latitudes are associated with the subtropical anticyclone and the large-scale descent of air from high-altitude currents moving toward the poles. After reaching the earth’s surface, this air spreads toward the equator as part of the prevailing trade winds or toward the poles as part of the westerlies. The belt in the Northern Hemisphere is sometimes called the “calms of Cancer” and that in the Southern Hemisphere the “calms of Capricorn”. The consistently warm, dry, and sunny conditions of the horse latitudes are the main cause for the existence of the world’s major non-polar deserts, such as the Sahara Desert in Africa, the Arabian and Syrian deserts in the Middle East, the Mojave and Sonoran deserts in the southwestern United States and northern Mexico, all in the Northern Hemisphere; and the Atacama Desert, the Kalahari Desert, and the Australian Desert in the Southern Hemisphere.
6. The Roaring Forties
The Roaring Forties are strong westerly winds found in the Southern Hemisphere, generally between the latitudes of 40 and 50 degrees. The strong west-to-east air currents are caused by the combination of air being displaced from the Equator towards the South Pole and the Earth’s rotation, and there are few landmasses to serve as windbreaks.
The Roaring Forties were a major aid to ships sailing the Brouwer Route from Europe to the East Indies or Australasia during the Age of Sail, and in modern usage are favoured by yachtsmen on round-the-world voyages and competitions. The boundaries of the Roaring Forties are not consistent, and shift north or south depending on the season. Similar but stronger conditions occur in more southerly latitudes and are referred to as the Furious Fifties and Shrieking or Screaming Sixties. Hot air rises at the Equator and is pushed towards the poles by cooler air travelling towards the Equator (an atmospheric circulation feature known as the Hadley Cell). At about 30 degrees from the equator, the outward-travelling air sinks to lower altitudes, and continues toward the poles closer to the ground (the Ferrel Cell), then rises up again from about 60 degrees as the air joins the Polar vortex. This travel in the 30 to 60 degree zone combines with the rotation of the earth to move the air currents from west to east, creating westerly winds.
Unlike the northern hemisphere, the large tracts of open ocean below 40th parallel south (interrupted only by Tasmania, New Zealand, and the southern part of South America) mean that higher wind speeds — the Roaring Forties — can develop. Similar but stronger wind conditions are prevalent closer to the South Pole; these are referred to as the “Furious Fifties” (50 to 60 degrees south), and the “Shrieking” or “Screaming Sixties” (below 60 degrees south). The latitude ranges for the Roaring Forties and similar winds are not consistent, shifting towards the South Pole in the southern summer, and towards the Equator in the southern winter.
During the Age of Sail, ships travelling from Europe to the East Indies or Australasia would sail down the west coast of Africa and round the Cape of Good Hope to use the Roaring Forties to speed their passage across the Indian Ocean, then on the return leg, continue eastwards across the Pacific Ocean and under Cape Horn before sailing up the east coast of the Americas to home. It was first used by Dutch explorer Hendrik Brouwer in his Brouwer Route, discovered in 1611, which effectively halved the duration of the trip from Europe to Java. “To run the easting down” was the phrase used to describe the fast passages achieved in the Roaring Forties. Round-the-world sailors also take advantage of the Roaring Forties to speed travel times, in particular those involved in record attempts or races.
7. Solar Eclipse
As seen from the Earth, a solar eclipse is a type of eclipse that occurs when the Moon passes between the Sun and Earth, and the Moon fully or partially blocks (“occults”) the Sun. This can happen only at new moon, when the Sun and the Moon are in conjunction as seen from Earth in an alignment referred to as syzygy.
8. Lunar Eclipse
A lunar eclipse occurs when the Moon passes directly behind the Earth into its umbra (shadow). This can occur only when the sun, Earth and moon are aligned (in “syzygy”) exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can occur only the night of a full moon.
9. Aurora Borealis (Northern Lights)
The Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights) are the result of electrons colliding with the upper reaches of Earth’s atmosphere. (Protons cause faint and diffuse aurora, usually not easily visible to the human eye.) The bright dancing lights of the aurora are actually collisions between electrically charged particles from the sun that enter the earth’s atmosphere. The lights are seen above the magnetic poles of the northern and southern hemispheres.
10. Jet Streams
Jet streams are fast flowing, narrow, meandering air currents found in the upper atmosphere or in troposphere of some planets, including Earth. The main jet streams are located near the altitude of the tropopause. The major jet streams on Earth are westerly winds (flowing west to east). Their paths typically have a meandering shape. Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet. The strongest jet streams are the polar jets, at 9–12 km (30,000–39,000 ft) above sea level, and the higher altitude and somewhat weaker subtropical jets at 10–16 km (33,000–52,000 ft). The Northern Hemisphere and the Southern Hemisphere each have a polar jet and a subtropical jet. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round. Jet streams are the product of two factors: the atmospheric heating by solar radiation that produces the large scale Polar, Ferrel, and Hadley circulation cells, and the action of the Coriolis force acting on those moving masses. The Coriolis force is caused by the planet’s rotation on its axis. On other planets, internal heat drives their jet streams. The Polar jet stream forms near the interface of the Polar and Ferrel circulation cells; while the subtropical jet forms near the boundary of the Ferrel and Hadley circulation cells.
11. El Niño
El Niño is the warm phase of the El Niño Southern Oscillation (commonly called ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (between approximately the International Date Line and 120°W), including off the Pacific coast of South America. El Niño Southern Oscillation refers to the cycle of warm and cold temperatures, as measured by sea surface temperature, SST, of the tropical central and eastern Pacific Ocean. El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. The cool phase of ENSO is called “La Niña” with SST in the eastern Pacific below average and air pressures high in the eastern and low in western Pacific. The ENSO cycle, both El Niño and La Niña, causes global changes of both temperatures and rainfall. Mechanisms that cause the oscillation remain under study. Developing countries dependent upon agriculture and fishing, particularly those bordering the Pacific Ocean, are the most affected. In American Spanish, the capitalized term “El Niño” refers to “the boy”, so named because the pool of warm water in the Pacific near South America is often at its warmest around Christmas. “La Niña”, chosen as the ‘opposite’ of El Niño, literally translates to “the girl”.