Sky coordinates and observations

Identify the direction while standing on the earth, and understand the rise and fall, field of view rotation, refraction, and conversion of star chart coordinates.

First look at the altitude angle and azimuth, then understand the angular distance in the field of view, and finally check the coordinate correction.

Let’s look at the phenomena in life first →

• How high should you raise your head when you see a star?

  Turn the abstract "elevation angle" into the action of raising your head while standing on the ground.

• I found it on the star map, but I couldn’t find it when I looked up: where should I look?

  Turn around first, then look up. Turn the address of the star catalog into an action you can follow.

• On a slightly flat earth model, does the "vertical foot" necessarily pass through the center of the earth?

  First stand on a smooth reference ellipsoid and compare the direction of the vertical surface with the line connecting the center of the earth.

• It's the same one degree difference on the map, so why are the distances different?

  The longitude lines meet at the poles, and the earth is not a perfect sphere: the map scale cannot be directly expressed as kilometers.

• The stars haven't moved, so why do the address numbers change when I change the star map?

  In the same direction, the celestial equator, ecliptic or galactic plane can be used as the starting point for measuring angles.

• Why do some stars never set all night long?

  Standing on the ground and looking at a circle of stars, the horizon is the boundary between "visible" and "blocked".

• How to calculate the time on the sunrise table step by step from the three-day ephemeris?

  Put the movement of the celestial body and the rotation of the earth on the same time line to find the real rising, mid-setting and setting events.

• Why do the patterns on the moon look crooked when I take pictures of it?

  If the phone is held upright, the moon surface pattern may be tilted: first distinguish between "above you" and "north of the celestial sphere".

• Why is the ecliptic in the sky sometimes slanted and sometimes steeper?

  The horizon follows where you stand, and the ecliptic follows the celestial sphere; as the earth rotates, the angle between the two changes.

• Why are there two "north directions" for the same sky photo?

  The equatorial grid and the ecliptic grid use different north poles. Choose a coordinate system before marking the direction.

• Why does the sunset go down diagonally and sometimes appear steeper?

  Fix the declination of a celestial body and see how the path caused by the Earth's rotation crosses your horizon.

• The star looks higher. Is it really lifted by the air?

  Neither the people nor the stars have been moved away, what the air changes is the direction in which the light reaches the eyes.

• If the weather changes, does the position correction of the same low-altitude star also change?

  In addition to how high a star is, air pressure and temperature also affect how light bends in the air.

• Are the two stars that appear to be next to each other really neighbors?

  First use your two lines of sight to measure: How much angle does it need to turn to look from one star to another?

• When two planets pass by each other, how do you find the closest moment?

  Don’t directly connect the daily distances into a parabola; first find where they are at each moment.

• Orion's belt is like a straight line, is it really not curved at all?

  Using three lines of sight, check how straight the "line" is.

• To capture three stars together in a photo, how wide must the field of view be?

  Try tightening a circular frame and see who hits the edge first.

• For the same star, why are there not enough addresses on the old star map?

  Only by separating the movement of the stars from the rotation of the coordinate grid can the year of the star catalog be read.

• The old star catalog says B1950, can it be used directly as J2000?

  Check the units of the coordinate ruler and the self-propelled axis first, and then decide how to convert.

• Will the "coordinate ruler" of the starry sky sway slightly on its own?

  See clearly the average direction and small periodic changes of the earth's axis, and distinguish between "the stars are moving" and "the ruler is moving".

• The star catalog records only one position, so why does the telescope need to make small corrections?

  Treat "the star is moving by itself", "the observer is moving" and "the coordinate ruler is changing" separately.

• For the same star, why does the old star catalog add one less optical aberration?

  It's not about counting less physics, but it's about avoiding counting twice the E item that's already hidden in the old star catalog.

• With the same comet orbit, why do the numbers change when I change the star map?

  What is being changed is the coordinate ruler, not moving the comet's orbit elsewhere.