# Right Ascension and Declination to the Horizon

• Philosophaie
In summary, the conversation discusses the difference in degrees of Right Ascension (RA) and Declination (DEC) when looking eastward from a certain longitude and latitude while on a ship in the middle of the ocean. It is mentioned that the difference in declination depends on the height, while the difference in right ascension also depends on the height and declination. The conversation also mentions drawing a sphere and a tangent to determine the horizon at a certain height, and that the horizon is at distance 0 at sea level. It is noted that the values for RA and DEC at sea level are dependent on the time of year, time of day, and longitude and latitude.

#### Philosophaie

Let's just say I was on a ship in the middle of the ocean. I am at a certain Longitude and Latitude with a Right Ascension (RA0) and Declination (DEC0) looking straight up into the heavens. If I look East how many degrees (RA and DEC) difference from my initial location to the horizon? Also in the other directions also.

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Hi Philosophaie!

Depends on height.

The difference in declination depends only on height.

The difference in right ascension depends on height and on declination.

Draw a sphere, stick a pin in it of height h, and draw a tangent.

The h=0 at mean sea level. You would only have the radius of the Earth at the specified longitude and latitude. I just want the extents of the RA and DEC window at sea level just a rough estimate.

At h = 0, the horizon is at distance 0.

The Right Ascension of the horizon at h=0 is not zero. The distance will always take into consideration the radius of the Earth in its calculations plus h. The value is dependant upon the time of year and time of day in addition to the Longitude and Latitude.

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## 1. What is Right Ascension and how is it calculated?

Right Ascension (RA) is the angular distance of a celestial object measured eastward along the celestial equator from the vernal equinox. It is one of the coordinates used in the equatorial coordinate system to locate objects in the sky. It is typically measured in hours, minutes, and seconds and its value changes as the Earth rotates. RA is calculated using the sidereal time and the object's hour angle.

## 2. What is Declination and how is it measured?

Declination (DEC) is the angular distance of a celestial object measured north or south of the celestial equator. It is also a coordinate in the equatorial coordinate system and is measured in degrees, minutes, and seconds. The declination of a celestial object can be measured using a declination circle or by calculating it from the object's altitude and azimuth coordinates.

## 3. How do Right Ascension and Declination relate to the horizon?

Right Ascension and Declination are coordinates used to locate celestial objects on the celestial sphere. These coordinates do not directly relate to the horizon, but they can be used to determine the altitude and azimuth of an object, which can then be used to calculate its position relative to the horizon.

## 4. How does the Earth's rotation affect Right Ascension and Declination?

The Earth's rotation causes the positions of celestial objects to appear to change in the sky. This affects both Right Ascension and Declination, as they are both based on the Earth's rotation. As the Earth rotates, the Right Ascension and Declination of an object will change, allowing us to track its movement in the sky.

## 5. Can you use Right Ascension and Declination to locate objects in the night sky?

Yes, Right Ascension and Declination can be used to locate objects in the night sky. By using a star chart or a computer program, you can determine the coordinates of an object and use them to find it in the sky. However, these coordinates are not as useful for stargazing as other coordinates, such as altitude and azimuth, as they require more calculations and are more affected by the Earth's rotation.