Astronomical Concepts

amateur  astronomy series #5

Celestial sphere: Inside of an imaginary sphere, with the  Earth at it's center, upon which all celestial bodies are assumed to be projected.

The Earth rotating within a relatively small-diameter Earth-centered celestial sphere. Depicted here are stars (white), the ecliptic (red), and lines of right ascension and declination (green) of the equatorial coordinate system.

The most obvious behavior of the celestial sphere is it's apparent daily east to west rotation, due to the axial spin of the earth.

The point which the stars appear to move around is one of the celestial poles.

The  celestial equator is the line around the celestial sphere that is half way between the celestial poles

The Altitude/ azimuth coordinate system can be used to describe a direction of view (The Azimuth angle) and a height in the sky (the Altitude angle).

The azimuth angle is measured clockwise round from due north. Hence North itself is 0°, East 90°, South-West 135° and so on. The altitude angle is measured up from the horizon.

Looking directly up (at the zenith) would be 90°, half way between the zenith and the horizon is 40° and so on. The point opposite zenith is called  nadir.

The altitude and azimuth values for an object in the sky change with time and location of the observer.

The celestial latitude and longitude
right ascension (α) or hour angle (h)  (horizontal) and declination (δ) (vertical)

RA/DC coordinate system uses two angles to describe positions in the sky. These angles are measured from standard points on the celestial sphere.

Right ascension and declination are to the celestial sphere what longitude and latitude are to terrestrial map makers.

celestial pole: The northern celestial pole has a declination of 90°, the celestial equator has a declination of 0° and the southern celestial pole has a declination of -90°.

Right ascension is measured as an angle round from a point in the sky known as the first point of Aries, in the same way that longitude is measured around the from Green witch. One hour of RA= 15° of arc

This was an accurate description of it's position thousands of years ago, but precaution has now carried it into the constellation of Pisces.

Some notes:

ecliptic: Apparent yearly path of the sun against the background of stars caused by the earth's orbital motion. Because of the 23.5°, "obliquity of ecliptic'. The annual path of the sun takes it through the familiar 12 constellations of the zodiac.

zodiac: Belt of constellations, roughly 8° on either side of the ecliptic, through which the sun, moon and planet appears to pass. The zodiac includes 12 familiar constellations, named after constellations they contained at the time of ancient Greeks.

In astrology zodiac is divided into 12 equal signs, each 30° long.

Constellation  are groupings of stars that are visually close to one another in the sky.

Polaris: A pole star is a visible star, preferably a prominent one, that is approximately aligned with the Earth's axis of rotation; that is, a star whose apparent position is close to one of the celestial poles, and which lies approximately directly overhead when viewed from the Earth's North Pole or South Pole. A similar concept also applies to other planets than the Earth. In practice, the term Pole Star usually refers to Polaris, which is the current northern pole star, also known as the North Star.

The south celestial pole currently lacks a bright star like Polaris to mark its position. At present, the naked-eye star nearest to this imaginary point is the faint Sigma Octantis, which is sometimes known as the South Star.

# In 14,000 years time Polaris will be nearly 47° away from the celestial pole.

# Polaris, North Star 90°

# Sigma Octantis -90 °

sidereal day and sidereal time (360° rotation, i.e. 23 hrs, 56 mins, 4.091 secs) and solar day and solar time (more than 360°)

Right ascension can be measured in angular terms. However, it is more common to use units of time (hours, minutes, seconds or h m s).

Astronomer measure time by the rotation of the Earth relative to the star, that is exactly 360° of axial rotation, rather than to the sun. Their sidereal time scale has a day of 24 sidereal hours, which in terms of civil time is 24 hours, 56 minutes and 4.091 seconds long.

The beginning of the sidereal day is when the First Point of Aries lies on the Meridian, the great circle of linking the north and south points and passing directly overhead. After 24 sidereal ours it will be in that position again. At 1 h sidereal time the sky will have rotated 15°.

Angles are measured in degree. The degree is divided in 60 minutes of arc (also known a arc-minutes). Each minute of arc is further subdivided i sixty seconds of arc (or arc-seconds). Thus one degree is equal to 3600 seconds of arc (further finer grades, as milli arc-seconds i.e. one thousandth of an arc-second).

Notation

90°  15'  12"

               12.432"

90.2533° 

Astronomical system of units

# An astronomical unit (abbreviated as AU, au, a.u., or ua) is a unit of length equal to exactly 149,597,870,700 metres (92,955,807.273 mi)^[1] or approximately the mean Earth–Sun distance by the best current (2009) estimate of the International Astronomical Union (IAU)

# Light year: speed of light 300,000 kms/second, i.e. 9.5 trillion kms= 1 light year
( the speed of light in a vacuum is defined as c₀ = 299,792,458 m/s in a vacuum, in accordance with the SI units.)

# Parsec, the distance of an object that has an annual parallax of one second of arc.

The astronomical distance unit parsec uses the AU as a baseline and an angle of one arcsecond for parallax. 1 AU and 1 pc not to scale.

Transits of Venus across the face of the Sun were, for a long time, the best method of measuring the astronomical unit, despite the difficulties (here, the so-called "black drop effect") and the rarity of observations.

*Note: all pictures thankfully shared from various sources..























*Note: all pictures thankfully shared from various sources..