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Astronomy I – Vocabulary you need to know: Altitude – Angular distance above or below the horizon, measured along a vertical circle, to the celestial object. Angular measure – Measurement in terms of angles or degrees of arc. An entire circle is divided into 360º, each degree in 60´ (minutes), and each minute into 60´´ (seconds). This scale is used to denote, among other things, the apparent size of celestial bodies, their separation on the celestial sphere, etc. One example is the diameter of the Moon’s disc, which measures approximately 0.5º= 30´. Azimuth – The angle along the celestial horizon, measured eastward from the north point, to the intersection of the horizon with the vertical circle passing through an object. Big Bang Theory – States that the universe began as a tiny but powerful explosion of space-time roughly 13.7 billion years ago. Cardinal points – The four principal points of the compass: North, South, East, West. Celestial equator – Is the great circle on the celestial sphere defined by the projection of the plane of the Earth’s equator. It divides the celestial sphere into the northern and southern hemisphere. The celestial equator has a declination of 0º. Celestial poles – Points above which the celestial sphere appears to rotate. Celestial sphere – An imaginary sphere of infinite radius, in the centre of which the observer is located, and against which all celestial bodies appear to be projected. Constellation – A Precisely defined part of the celestial sphere. In the older, narrower meaning of the term, it is a group of fixed stars forming a characteristic pattern. The names and boundaries of the constellations are mostly derived from ancient mythology. The heavens are divided into 88 constellations. Declination – Analogous to latitude on Earth it is the angular distance of a celestial body from the celestial equator. It is measured north and south of the celestial equator. Ecliptic – The apparent annual path of the Sun on the celestial sphere. The Sun traverses the whole ecliptic in a year. Horizon (astronomical) – A great circle on the celestial sphere 90° from the zenith. Hour angle – The angle measured westward along the celestial equator from the local meridian to the hour circle passing through an object. Meridian - An imaginary north-south line in the sky that passes through the observer's zenith. Precession – The small wobbling motion around the Earth's axis that the Earth makes as it spins (just like the wobbling motion of a spinning top). This causes the Earth's axis to point in a slightly different direction over time; Polaris is our pole star now, but this is not permanent because of precession. Precession is caused by the gravitational influences of the Sun and the Moon and has a period of 25,800 years. Right ascension – A celestial coordinate analogous to longitude on Earth and measured around the celestial equator from the vernal equinox. Together with declination, they give the position of a point on the celestial sphere. It is expressed in hours (from 0 to 24), minutes and seconds. Zenith - An observer's zenith is the point directly overhead.