The celestial sphere refers to a large fictional sphere. Infinite With the world at the center Former humans imagined that The Earth is wrapped in a celestial sphere which is adorned with a star (Star) and understands that these stars are at the same distance from Earth. Equal to the radius of the celestial sphere 1 celestial sphere spins the Earth 1 day, allowing us to see stars moving along the celestial sphere at a rate of 15 degrees per hour (360 degrees°/ 24 hours = 15°).

The sun moves in the sky along the solar path. The 12 constellations that follow the solar path are known as the zodiac sign. Each month the sun moves through the zodiac sign. Which is the constellation of the month. For example, in September, the sun moves in front of the constellation Virgo. The sun moves through the twelve signs of the zodiac. It lasts for 1 year. For this reason, one year is 12 months (360°/ 12 months = 30 days). Humans observe the sun, moon and five bright stars visible to the naked eye: Mars, Mercury, Jupiter, Venus and Saturn. With unstable direction and speed So these seven stars are called planets, which are the names of the seven days of the week.

We can see that humans observe the movement of the sun, moon, stars and planets. In the sky sphere Leads to clock and calendar timing Which can be considered as the wisdom of humans since ancient times However, the study of the celestial sphere is not a study of the past. But is a study of current facts Which requires both views View from the world that sees the celestial sphere moving around the world and a view from space in which the Earth revolves around itself and orbits the sun.

**Horizon** **coordinates** **system **

Horizontal coordinates are a coordinate system used to measure the position of celestial objects. By taking the observer’s body as the center of the celestial sphere With the imaginary points and lines on the celestial sphere. The four directions consist of North, Earth, South, and West. When facing north The back is south. Left is west The right is the east.

- The Zenith is the highest point of the celestial sphere. Above the observer.
- The bottom point (Nadir) is the lowest position of the celestial sphere. Under the feet of the observer.
- Horizon means the horizon line visible to the ground. Or, in other words, The large circle on the celestial sphere 90 degrees above the head.° With the main axis of the horizon system.
- The Meridian is an imaginary line on the north-south circular sphere. Which is pulled over the point of the head.

Angle measurements in the horizon coordinate system consist of direction and elevation angles.

- Azimuth (Azimuth) angles horizontally. Which is measured from the north (0° ) Follow the horizon clockwise To the east (90°) South (180°) West (270°) And return to the north (360°) One more time Therefore, the direction angle has a value between (0° – 360°)
- Altitude is the vertical angle Which counted from the horizon (0°) up to the point of overhead (9 0°). Therefore, the elevation angle is between (0° – 90°)

Looking at the position of the stars in Figure 2, the star has a coordinate of the horizon in the direction of 25 0.° elevation angle 45° To measure the distance of the stars in the sky is the angular distance.

**Angular Distance**

To measure the distance of stars and celestial objects We cannot measure the distance in meters. Or kilometers directly if We do not know that those objects How far is it from us? Therefore, the measurement of astronomical distances Therefore, it is preferable to measure it in angular distance. For example, we say that Star A is 5 degrees away from star B or how many degrees the moon is. Which is to tell the distance and the size in total angular. That the measured angular distance Is the distance that appears only But in reality, Stars A and Stars B may not be equal to us. Or may be really equal distances from us because the stars that we see in the sky we see only 2 dimensions, but the depth dimension that we can not observe

Simple angular distance measurement.

In measuring angular distances if you want detailed and accurate values Must use very sophisticated equipment for measuring But if you only need the approximate value We can measure angular distances using only our own hands and fingers. For example, if we extend our hands, raise our thumb and little finger By fully stretching your arms The width of both fingers relative to the angle in the sky. Will get an angle of about 18 degrees. If two stars are at a distance from this width, it means Both stars are 18 degrees apart.

On a night with a full moon. Let us try to hold a little finger and stretch the arms out as far as possible. Prostrate the little finger and the moon. We will find that our little finger fits the moon perfectly. We say that the moon is ” angular diameter ” (Angular Diameter) equal to 1/2 degree angular diameter is. Angular distance measured between edges Of the moon itself The angular size of an object depends on the distance of the object to the observer. And the actual diameter of the object.

Imagine that the ball is 1 meter away from us. Let us measure the angular size of the ball. Then move the ball 3 times as far as the angular size decreases to 1 in 3 of the previously measured size. Therefore, “angular size” is the ratio of the actual size to the distance of the object.