What Are Ra And Dec

What are RA and DEC?

If you are a beginner astronomer, you will inevitably come across the RA and DEC systems. When you are just getting started, these concepts can get quite overwhelming.

RA and DEC are integral to astronomy. They are invisible calibration that helps you located celestial objects anywhere in the deep sky.

For a successful endeavor as an astronomer, it is vital to learn about these coordinates. In this section, we present you with a clear understanding of the RA and DEC coordinates.

The Equatorial Coordinate System

Most astronomers prefer the Equatorial coordinate system to locate celestial bodies. This system is more straightforward than the horizontal coordinate order.

The EQ coordinates are independent of the observation time and location. This entails that you don’t require multiple coordinates for each object.

You can apply the same coordinate for different locations in the celestial sphere. The simplicity of the EQ system makes it so desirable for astronomers.

The principle of the EQ coordinate system is similar to latitude and longitude coordinates on Earth. Hence, lines of latitude are equivalent to DEC (declination). And Longitude is for RA (right ascension).

The analogy makes the positioning via the coordinates easy to implement. But that is only true in theory.

The EQ coordinate system is based on the positioning of Earth in space. This orientation is subjected to change in 26000 years. A direct effect of the earth axis’s precession.

This makes it necessary to include the concept of epoch to the coordinates. Here is an example for better understanding.

The Einstein cross was previously located at DEC- +03005’ and RA- 22h 37m. This coordinates are based on epoch B1950.

Now let’s look at the same coordinates in epoch J2000.0. The RA is 22h 37m, and the DEC is +03o21’.

This helps us understand the changes in coordinates better. The object does not move, but its coordinates do.

What is RA?

RA or right ascension is a measure of east and west on the celestial sphere. DEC or declination is the measure of north and south in the same sphere.

As the Earth rotates to the east, the sky moves to the west during the day. Hence, RA is calibrated in minutes and seconds.

The celestial sphere is positioned at one hour of the west. Considering the 360 degrees of rotation, one hour of RA is equivalent to 15 degrees rotation.

The circles of RA converge on the celestial poles of both the north and the east. The Greenwich meridian is the longitude line dividing the east from the west.

The same Greenwich line, when depicted on the RA axis is present as 00h 00m 00s. The Vernal Equinox is the intersection point of the celestial meridian and celestial equator lines.

What is DEC?

It is clear now that RA is the measurement of the celestial eastern and western. Then, DEC or declinations are the northern and southern measurements of the sky.

Declination is analogous to latitude on the Earth. Hence, it is calibrated in degrees, minutes, and seconds. The measurement is positive for north and negative for the south.

Similar to RA, the celestial equator is present at 0 degrees DEC. The northern and the south poles are at +90 to -90 degrees, respectively.

One of the essential celestial markings in astronomy is the Polaris Star. The star is present at +89.2 degrees declination.

In terms of declination, the equator divides the sphere into northern and southern hemispheres. It is easy to figure out how high a star travels, with its declination coordinates.

What makes it important to learn RA and DEC?

As we mentioned before, the Earth’s axis is essentially a bit ‘wobbly.’ This causes a direct effect to the celestial coordinates as they change periodically.

The equinox coordinates drift further west at a rate of 50.3 arc seconds. As this drift occurs, there is a sure shift in the celestial coordinate grid as well.

This is a big reason behind the routine upgrades of celestial catalogues and programs. The enhancements are made in line with the most recent epoch standards.

The upgrades are conducted every 50 years. In line with that system, the next significant update of the celestial coordinates and programs will happen in 2050.

Both RA and Dec are essentials in astronomy. But they become even more critical for go-to telescope users.

The Difference between arc minutes and minutes

You understand this better with a little knowledge of geometry. The circle makes a full 360 degrees.

Each degree is further divided into 60 arc minutes. And one arc minute is then divided into 60 arc seconds.

Meaning each degree has 3600 arc seconds. A minute devoid of the arc is the distance moved by the object due to the Earth’s rotation.

The celestial equator represents one hour into 15 degrees. When you move farther away from the equator, the distance moved by an object is minimal.

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