Query: A South African paper has found a 21 year cycle synchronous with the solar cycle (Alexander 2007).

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Little Ice Age <=SEP=> The Little Ice Age ( LIA ) was a period of cooling that occurred after the Medieval Warm Period . Although it was not a true ice age , the term was introduced into scientific literature by François E. Matthes in 1939 . It has been conventionally defined as a period extending from the 16th to the 19th centuries , but some experts prefer an alternative timespan from about 1300 to about 1850 . Climatologists and historians working with local records no longer expect to agree on either the start or end dates of the period , which varied according to local conditions .   The NASA Earth Observatory notes three particularly cold intervals : one beginning about 1650 , another about 1770 , and the last in 1850 , all separated by intervals of slight warming . The Intergovernmental Panel on Climate Change Third Assessment Report considered the timing and areas affected by the Little Ice Age suggested largely-independent regional climate changes rather than a globally-synchronous increased glaciation . At most , there was modest cooling of the Northern Hemisphere during the period .   Several causes have been proposed : cyclical lows in solar radiation , heightened volcanic activity , changes in the ocean circulation , variations in Earth 's orbit and axial tilt ( orbital forcing ) , inherent variability in global climate , and decreases in the human population .

Ice age <=SEP=> An ice age is a period of long-term reduction in the temperature of Earth 's surface and atmosphere , resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers . Within a long-term ice age , individual pulses of cold climate are termed `` glacial periods '' ( or alternatively `` glacials '' or `` glaciations '' or colloquially as `` ice age '' ) , and intermittent warm periods are called `` interglacials '' . In the terminology of glaciology , ice age implies the presence of extensive ice sheets in both northern and southern hemispheres . By this definition , we are in an interglacial period -- the Holocene -- of the ice age . The ice age began 2.6 million years ago at the start of the Pleistocene epoch , because the Greenland , Arctic , and Antarctic ice sheets still exist .

Leap year <=SEP=> A leap year ( also known as an intercalary year or bissextile year ) is a calendar year containing one additional day ( or , in the case of lunisolar calendars , a month ) added to keep the calendar year synchronized with the astronomical or seasonal year . Because seasons and astronomical events do not repeat in a whole number of days , calendars that have the same number of days in each year drift over time with respect to the event that the year is supposed to track . By inserting ( also called intercalating ) an additional day or month into the year , the drift can be corrected . A year that is not a leap year is called a common year .   For example , in the Gregorian calendar , each leap year has 366 days instead of the usual 365 , by extending February to 29 days rather than the common 28 . Similarly , in the lunisolar Hebrew calendar , Adar Aleph , a 13th lunar month , is added seven times every 19 years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons . In the Baha'i Calendar , a leap day is added when needed to ensure that the following year begins on the vernal equinox .   The name `` leap year '' probably comes from the fact that while a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next , the day of the week in the 12 months following the leap day ( from March 1 through February 28 of the following year ) will advance two days due to the extra day ( thus `` leaping over '' one of the days in the week ) . For example , Christmas Day fell on a Tuesday in 2001 , Wednesday in 2002 , and Thursday in 2003 but then `` leapt '' over Friday to fall on a Saturday in 2004 .   The length of a day is also occasionally changed by the insertion of leap seconds into Coordinated Universal Time ( UTC ) , owing to the variability of Earth 's rotational period . Unlike leap days , leap seconds are not introduced on a regular schedule , since the variability in the length of the day is not entirely predictable .

Moon <=SEP=> The Moon is an astronomical body that orbits planet Earth , being Earth 's only permanent natural satellite . It is the fifth-largest natural satellite in the Solar System , and the largest among planetary satellites relative to the size of the planet that it orbits ( its primary ) . Following Jupiter 's satellite Io , the Moon is second-densest satellite among those whose densities are known .   The average distance of the Moon from the Earth is 384400 km , or 1.28 light-seconds .   The Moon is thought to have formed about 4.51 billion years ago , not long after Earth . There are several hypotheses for its origin ; the most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia .   The Moon is in synchronous rotation with Earth , always showing the same face , with its near side marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters . It is the second-brightest regularly visible celestial object in Earth 's sky , after the Sun , as measured by illuminance on Earth 's surface . Its surface is actually dark , although compared to the night sky it appears very bright , with a reflectance just slightly higher than that of worn asphalt . Its prominence in the sky and its regular cycle of phases have made the Moon an important cultural influence since ancient times on language , calendars , art , and mythology .   The Moon 's gravitational influence produces the ocean tides , body tides , and the slight lengthening of the day . The Moon 's current orbital distance is about thirty times the diameter of Earth , with its apparent size in the sky almost the same as that of the Sun , resulting in the Moon covering the Sun nearly precisely in total solar eclipse . This matching of apparent visual size will not continue in the far future . The Moon 's linear distance from Earth is currently increasing at a rate of 3.82 ± per year , but this rate is not constant .   The Soviet Union 's Luna programme was the first to reach the Moon with uncrewed spacecraft in 1959 ; the United States ' NASA Apollo program achieved the only crewed missions to date , beginning with the first crewed lunar orbiting mission by Apollo 8 in 1968 , and six crewed lunar landings between 1969 and 1972 , with the first being Apollo 11 . These missions returned over 380 kg of lunar rocks , which have been used to develop a geological understanding of the Moon 's origin , the formation of its internal structure , and its subsequent history . Since the Apollo 17 mission in 1972 , the Moon has been visited only by uncrewed spacecraft .