# How do you calculate declination and Right Ascension from Earth Coordinates?

• Brynjolf
In summary, the coordinates 40.23°N and 15.89°E do not directly determine the declination and Right Ascension of a point in the night sky. These coordinates only provide the observer's location on Earth, while RA and Dec are measurements of a point in the sky. To calculate RA and Dec, one must use a star map or atlas, which shows the coordinates of different objects in the sky. The only variable is whether the object is above or below the horizon at a specific time and location.
Brynjolf
Say a person is positioned here: 40.23°N and 15.89°E and was examining the night sky.

How do you calculate the declination and Right Ascension from that location's coordinates?

I know the RA is measured in hours up to 24 and Declination in degrees.

Any ideas?

r m williams
Your question doesn't make much sense. Declination and Right Ascension are coordinates of a point on the sky. So it depends not on where you are on Earth, but on where in the sky you are looking. At any time, you can see half the sky, so you could be looking at a wide range of RA and Dec coordinates.

Hi Brynjolf

welcome to PF :)

yes I have to agree with physguy on that one. Starmaps/atlases are produced showing RA and Dec co-ordinates of the sky. You are able to use that star map anywhere in the world without having to do any conversion.

The only thing that varies is whether a particular RA and Dec co-ordinate is above or below your local horizon at a given time of the day/year from you current location

cheers
Dave

Straight up Azimith = 0 and Altitude = 90 at":

40.23°N and 15.89°E

RA = LMST

DEC = Latitude = 15.89°E

LMST (Local Mean Sidereal Time = GMST0 + time * 15 + Longitude)
GMST0 (Greenwich Mean Sidereal Time at Midnight)

Last edited:

To calculate declination and Right Ascension from Earth coordinates, we first need to understand the celestial coordinate system used in astronomy. The celestial coordinate system is similar to the Earth's coordinate system, with the equator being the celestial equator and the poles being the celestial poles. However, instead of longitude and latitude, we use Right Ascension (RA) and declination (Dec) to locate objects in the sky.

Right Ascension is measured in hours, minutes, and seconds, with 24 hours being equivalent to a full circle around the celestial equator. This means that one hour of RA is equal to 15 degrees of longitude on Earth. Declination is measured in degrees, with positive values being north of the celestial equator and negative values being south.

To calculate the declination and Right Ascension from a specific location on Earth, we need to use the concept of local sidereal time (LST). LST is the time it takes for the vernal equinox (a point on the celestial equator where the Sun crosses from south to north) to reach the meridian (an imaginary line that runs from the north celestial pole to the south celestial pole) at a specific location on Earth. It is measured in hours, minutes, and seconds, just like RA.

To calculate LST, we need to know the longitude of the location in question and the current Universal Time (UT). UT is the time measured at the Prime Meridian (0 degrees longitude) and is used as a standard reference for all locations on Earth. Once we have the LST, we can use it to calculate the RA and declination of any object in the sky.

For example, if a person is located at 40.23°N and 15.89°E, their longitude is 15.89°E, and we can use this to calculate LST. Let's say the current UT is 10:00 PM. To convert this to LST, we need to add the longitude to the UT. In this case, 15.89°E is equivalent to 1 hour and 3 minutes (15.89/15=1.06 hours). So the LST for this location at 10:00 PM UT would be 11:03 PM.

Now, let's say the person is examining the night sky and sees a star located at 20 degrees above the horizon and 10 degrees east of the meridian. We can

## What is declination and Right Ascension?

Declination and Right Ascension are two celestial coordinates used to pinpoint the location of an object in the sky. Declination is equivalent to latitude on Earth, while Right Ascension is similar to longitude.

## What are Earth Coordinates?

Earth Coordinates refer to the coordinates used to locate objects in the sky from the perspective of an observer on Earth. This includes the Earth's equator, poles, and the ecliptic plane.

## How do you calculate declination and Right Ascension from Earth Coordinates?

To calculate declination and Right Ascension from Earth Coordinates, you will need to know the object's altitude and azimuth, as well as the observer's latitude and longitude. These values can then be plugged into equations using trigonometry and the object's position can be determined.

## Why are declination and Right Ascension important in astronomy?

Declination and Right Ascension are important in astronomy because they provide a standardized way to locate objects in the sky. This allows astronomers to accurately track and study celestial objects.

## Are there any tools or software that can calculate declination and Right Ascension from Earth Coordinates?

Yes, there are various tools and software available that can calculate declination and Right Ascension from Earth Coordinates. Some popular options include planetarium software, online calculators, and astronomical handbooks.

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