By Reuben Storr

TidesTides

• Tides are the rise and fall of sea levels caused by the combined Tides are the rise and fall of sea levels caused by the combined effects of the rotation of the Earth and the gravitational forces effects of the rotation of the Earth and the gravitational forces made by the Moon and the Sun. The tides occur with a period of made by the Moon and the Sun. The tides occur with a period of approximately 12 and a half hours and are influenced by the approximately 12 and a half hours and are influenced by the shape of the near-shore bottom. shape of the near-shore bottom.

seasonsseasons

• A season is a division of the year, marked by changes in A season is a division of the year, marked by changes in weather, hours of daylight. Seasons result from the yearly weather, hours of daylight. Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution. In temperate Earth's axis relative to the plane of revolution. In temperate and polar regions, the seasons are marked by changes in the and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to go into hibernation variations of which may cause animals to go into hibernation or to migrate, and plants to be dormant. The four seasons are or to migrate, and plants to be dormant. The four seasons are summer,winter,autum and spring. summer,winter,autum and spring.

The big bang theoryThe big bang theory

• According to the Big Bang model, the universe According to the Big Bang model, the universe expanded from an extremely dense and hot state and expanded from an extremely dense and hot state and continues to expand today. A common analogy explains continues to expand today. A common analogy explains that space itself is expanding, carrying galaxies with it, that space itself is expanding, carrying galaxies with it, like raisins in a rising loaf of bread.like raisins in a rising loaf of bread.

equinoxequinox

• An equinox occurs twice a year, when the tilt of the An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards Earth's axis is inclined neither away from nor towards the Sun, the centre of the Sun being in the same plane the Sun, the centre of the Sun being in the same plane as the Earth's equator.An equinox happens each year at as the Earth's equator.An equinox happens each year at two specific moments in time (rather than two whole two specific moments in time (rather than two whole days), when there is a location on the Earth's equator days), when there is a location on the Earth's equator where the centre of the Sun can be observed to be where the centre of the Sun can be observed to be vertically overhead, occurring around March 20/21 and vertically overhead, occurring around March 20/21 and September 22/23 each year.September 22/23 each year.

eclipseseclipses

• As seen from the Earth, a solar eclipse occurs when As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon passes between the Sun and the Earth, and the Moon fully or partially covers the Sun as viewed the Moon fully or partially covers the Sun as viewed from a location on Earth. This can only happen during from a location on Earth. This can only happen during a new moon, when the Sun and Moon are in a new moon, when the Sun and Moon are in conjunction as seen from Earth. At least two, and up to conjunction as seen from Earth. At least two, and up to five, solar eclipses occur each year; no more than two five, solar eclipses occur each year; no more than two can be total eclipses. Total solar eclipses are can be total eclipses. Total solar eclipses are nevertheless rare at any particular location because nevertheless rare at any particular location because totality exists only along a narrow path on the Earth's totality exists only along a narrow path on the Earth's surface traced by the Moon's umbra.surface traced by the Moon's umbra.

The moon The moon • As the moon circles the Earth, the shape of the moon As the moon circles the Earth, the shape of the moon

appears to change; this is because different amounts appears to change; this is because different amounts of the illuminated part of the moon are facing us. The of the illuminated part of the moon are facing us. The shape varies from a full moon (when the Earth is shape varies from a full moon (when the Earth is between the sun and the moon) to a new moon between the sun and the moon) to a new moon (when the moon is between the sun and the Earth). (when the moon is between the sun and the Earth).

The moon cycleThe moon cycle

monthsmonths

• A month is a unit of time, used with calendars, which A month is a unit of time, used with calendars, which was first used and invented in Egypt, as a natural period was first used and invented in Egypt, as a natural period related to the motion of the Moon. Months are related to the motion of the Moon. Months are approximately 29.53 days. Synodic months, based on approximately 29.53 days. Synodic months, based on the Moon's orbital period, are still the basis of many the Moon's orbital period, are still the basis of many calendars today, and are used to divide the year.calendars today, and are used to divide the year.