How do we(or the NASAians) find out the distance between heavenly bodies in light years? Ho do they come to know that a particular body is n light years away from earth? Similarly, how did they come to know that sun is 8 light minutes away from us( is that right? )?

Good question! You always come up with real ‘head scratchers.’
Here’s what my dusty, elderly copy of Encyclopaedia Brittanica has to say from the historical
aspect: the speed of light and the measurement of distances traveled by light were determined by
experimentation and observation. Observation first.
Galileo wanted to determine if light occurred instantaneously or had a finite velocity so he had 2
men with lanterns stand several miles apart (with telescopes, if needed). The first man uncovered
his lantern and at the instant that the second man saw the light, he uncovered his lantern. Galileo
thought that there might be a time difference, thus proving that the propagation of light was not
instantaneous, as the ancients thought.
The experiment failed to be conclusive because of human reaction time and the fact that light
would travel the distance in a few millionths of a second.

An English astronomer, James Bradley, in 1728, found evidence of a finite velocity of light by
observing a uniform annual shift in the apparent position of all stars in the earth’s orbital velocity.
Bradley estimated the stellar aberration to be 20 seconds of arc, giving a value of 295,000 km/sec
for the velocity of light.

Experimentation included using rotating mirrors, rotating prisms, electro-optical shutters, high
frequency oscillators, cavity resonance, microwave interferometers, measurements of radio and
radar waves, beam splitters, phase shifters, and a whole family of other instruments that measure
X-rays, gamma rays and other radiation sources.

None of the above includes experiments and measurements done by satellite. Hope this helps.

Pretty interesting thing! But, I'm even more perplexed.
That was the case on earth, where people are everywhere! But what of space?

Off the top of my head I can think of two methods.

Parallax and Doppler shift methods but I suggest:

http://www.astro.ucla.edu/~wright/distance.htm

Wonderful site. I got all my answers. Thanks very much Tielhard!

Just wanted to verify, is sun 8 light minutes away from us?

"After a photon is created, it becomes absorbed and will be emitted in random directions. It will travel through more than 400,000 miles of plasma. It will take 30,000 years to make it to the surface of the sun. After this, it will take 8 minutes to get to Earth."

http://www.light-science.com/toilets.html

The sun is about 93,000,000 miles away, and light travels at about 186,000 miles a second. So yes, the distance would be about 8 1/3 light minutes, i.e. the light from the sun takes roughly 8 minutes and 20 seconds to reach the earth.

That raises another question in my mind. When an almanac says, for instance, that sunrise today is at 7:00 a.m., is that the time that the sun really “rises”, or is it the time that we see it rise.

Take a look at this site: http://aa.usno.navy.mil/faq/docs/RST_defs.html#top

Thanks.