Thursday, March 09, 2017

Adarsha’s Star



I believe I have given enough time to readers to examine AJ's answer and to discuss it. Over a hundred people have visited the post above. So, it is time now for me to comment on the answer.

AJ (Adarsha Joisa) is right on both counts - the main answer as well as the answer to the sub-question. I had referred to a list of 92 stars brighter than magnitude 2.49. One of these can be said to be “nearest” to Adarsha’s Direction, defined as the intersection of the local meridian and the ecliptic at 6 AM that day. But the star may not be very close to the direction, and may even be more than ten degrees away. 

So, let us change the way we define Adarsha’s Star: On a given day, it will be the star closest to Adarsha’s Direction provided it is in the list of the 170 brightest stars (Visit http://stars.astro.illinois.edu/sow/bright.html ). The dimmest stars in this list are of apparent magnitude 3.0.

The direction of the earth’s orbital velocity around the Sun changes even during the day. So, what is Adarsha’s Star on a given day at 6 AM in Tokyo may not be the one that is Adarsha’s Star at local time 6 AM in Los Angeles on the same day. I am not going to worry about refining the designation of Adarsha’s Star any further now, particularly when none of the 170 brightest stars may be within 5 degrees of Adarsha’s Direction at a given moment.

Let us ask what the purpose of this exercise is. We want to be able to point in a direction in the sky and say that it is the direction of earth’s orbital velocity. Ten degrees of accuracy is good enough for this purpose.

What about planets? The five planets visible to the unaided eye are useful in visualizing what we are hurtling towards at a given time. They have high visibility and are always close to the ecliptic.
Let us look at the sub-question. Many of the 92 stars above magnitude 2.49 are well away from the ecliptic. Since the ecliptic is fairly stable with respect to the stellar background, many of them have no chance in my lifetime, or yours, of being Adarsha’s Star!

What is Adarsha’s Direction today? At Bangalore, the Local Sidereal Time (LST)at 6 AM today (March 9, 2017) was 16 h 53 minutes, as per https://www.iiap.res.in/personnel/reks/software/javascript/calclst.php
(All points in the sky having Right Ascension = LST are on the local meridian, by definition). I looked up a sky map to find a point on the ecliptic which had the right ascension of 16 H 53 Minutes approximately.  I found that this point is within five degrees of the bright star Antares in the constellation Scorpius. That much accuracy will do for me. I am now very happy to know I am hurtling at 110,000 kilometers per hour, roughly towards Antares!
I hope that any policeman who stops me for speeding would have a sense of proportion, and not bother about an extra twenty kilometers per hour or so!    

3 comments:

Srinivasan Ramani said...

An astrologer may suggest a simpler method of finding out where we are headed. Look at the Zodiac Chart, and mark in which “house”, H, the Sun is located now. Count clockwise from H, and stop at the fourth house. That is what the earth seems to be moving towards.
This method is inaccurate because these charts are not geometrically accurate maps; they are symbolic ones.

Sakthivel said...

Thanks sir very interesting. Nice exercise. Am I right in saying that (from Coimbatore) at 6am I am hurling almost vertically up at 30km/s and at 6pm I will be falling down with the same speed.

Srinivasan Ramani said...

"Down" is the direction of the centre of the earth in relation to where you are. As the earth rotates round its axis, the "down" direction changes in relation to the stellar background. However, the earth's orbital velocity changes direction by only about half a degree between 6 AM and 6 PM on the same day. So, if you were hurtling towards Antares in the morning, you would continue to be hurtling more or less in the same direction in the evening, but you would be going feet first in the evening! Take care!