Definitions+&+Key+Words

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**Axial tilt**: Axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane. The orbital plane is the term that refers to a semi-flat disk made from all of the orbits of all of the planets that orbits line up with each other.

The Earth currently has an axial tilt of about 23.4°. The axis remains tilted in the same direction towards the stars throughout a year and this means that when a hemisphere (a northern or southern half of the earth) is pointing away from the Sun at one point in the orbit then half an orbit later (half a year later) this hemisphere will be pointing towards the Sun. This effect is the main cause of the seasons. The hemisphere that is tilted toward the Sun experiences more hours of sunlight each day. media type="youtube" key="DuiQvPLWziQ" height="283" width="378" align="left" 

**Celestial**: //adjective// positioned in or relating to the sky, or outer space as observed in astronomy. e.g. //a celestial body//

**Eclipse**: Eclipses occur when one celestial body passes into the shadow of another. A lunar eclipse occurs when the moon passes behind the earth so that the earth blocks the sun's rays from striking the moon.

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Eclipses do not occur every time there is a full or new moon because the moon's orbit around the Earth is inclined.

A //solar eclipse// occurs when the Earth passes through Moon’s shadow. They happen during a new moon.



**Lunar phase**: A lunar phase or phase of the moon is the appearance of the illuminated (lighted) portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun. One half of the lunar surface is always illuminated by the Sun (except during lunar eclipses), and hence is bright, but the portion of the illuminated hemisphere that is visible to an observer can vary from 100% (full moon: when the Moon is on the opposite side of the Earth from the Sun) to 0% (new moon: when the Moon, lies between Earth and the Sun).



**Orbit**: An orbit is a regular, repeating path of one body around another. It is a curved path called an ellipse. Earth’s orbit of the Sun is nearly circular. The Moon’s orbit of the Earth is almost circular. A comet’s orbit is very long and thin.

**Planet**: A body that orbits the Sun and which shines only by reflected sunlight.

**Planetary Seasons**: Planetary seasons are seasons that occur on other planets in our solar system. They are caused by two factors: axial tilt and variable distance from the sun.

**Satellite**: Any body in orbit around a larger body.

**Seasons**: Seasons are divisions of the year characterised by variations in the relative lengths of day and night and in the amount of heat received from the sun. Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution.

Practice your understanding of how seasons work with this learning object.

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**Solar System**: The system of planets, moons, comets and asteroids that are in orbit around the sun. Our Solar System is in the Milky Way Galaxy. It consists of an average-sized star we call the Sun and the eight planets, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.