# Connection between right ascension and time

• Wheelwalker
In summary, the conversation discusses the concepts of declination and right ascension in relation to celestial objects. The problem at hand involves determining the accuracy of a description of Rigel rising in the night sky in Cuba. The key to solving the problem is understanding the different 'clocks' used to define RA and how to factor in time and longitude. The final step is to calculate the Hour Angle using LMST, GMST, and the given RA.

#### Wheelwalker

I'm a physics major currently taking my first astro class. We're covering the basics at the moment but I am having trouble visualizing this question from our textbook. To preface this, I understand that declination is to the celestial sphere what latitude is to the Earth and RA is to the celestial sphere what longitude is to the earth. I also know that RA is measured from the vernal equinox. The problem references Hemingway's "The Old Man and the Sea" and describes a man in Cuba lay in his boat shortly after the sun set one September night and saw Rigel rising. I'm supposed to find what is incorrect about this. I'm fairly certain that Rigel wouldn't appear in the night sky until much later than the sun sets. Rigel's RA is 05h 14m. The longitude of Cuba (in a very general sense) is approximately 80 degrees west. But where do I go from here? Do I calculate how many hours away Cuba's longitude is from Rigel's RA? How do I factor in the time?

You need to determine what 'clock' is used to define RA, and the clock used by the 'Old Man'. The rest is easy.

So the 'clock' used to define RA is that 0h is at the vernal equinox, and goes up east from there. The clock used by the Old Man allow him to read the time at approximately sunset in Cuba in late September?

There is time in the Hour Angle:
$$LMST = GMST + time + Longitude/15$$$$HA = LMST - RA$$where LMST is Local Mean Sidereal Time in Hours. GMST is Greenwich Mean Sidereal time. The Sidereal Time above London, England. And time is in hours also.

## 1. What is right ascension and how is it related to time?

Right ascension is a celestial coordinate used to measure the east-west position of a celestial object in the sky. It is measured in hours, minutes, and seconds and is analogous to longitude on Earth. The connection between right ascension and time is that right ascension increases by 15 degrees for every hour of time that passes. This means that the right ascension of a celestial object can be used to determine its position in the sky at a specific time.

## 2. What is the significance of the zero point of right ascension?

The zero point of right ascension is defined as the point where the celestial equator intersects the ecliptic. This is known as the vernal equinox and is used as the starting point for measuring right ascension. It is significant because it allows for a consistent and standardized way to measure the positions of celestial objects in the sky.

## 3. How is right ascension different from declination?

Right ascension and declination are both celestial coordinates used to locate objects in the sky. Right ascension measures the east-west position, while declination measures the north-south position. Right ascension is measured in hours, minutes, and seconds, while declination is measured in degrees, minutes, and seconds. Together, these coordinates can pinpoint the exact location of a celestial object in the sky.

## 4. Can right ascension be converted to other units of measurement?

Yes, right ascension can be converted to other units of measurement such as degrees or radians. This can be done by converting the hours, minutes, and seconds of right ascension into decimal degrees or radians using a conversion formula. However, it is important to note that right ascension is most commonly used in its original units of hours, minutes, and seconds.

## 5. How does right ascension change over time?

Right ascension does not change over time for a specific celestial object. However, due to the Earth's rotation, the right ascension of celestial objects appears to shift throughout the night. This means that the right ascension of an object will change as the Earth rotates, but its actual position in the sky remains the same. This is why right ascension is closely related to time, as it can be used to track the movement of objects in the sky over time.