Laser Measure - Need Help

[cabinetman]

Got one of those laser measures as a gift, and have never really used it for anything meaningful. Then the other night, I had this brainstorm to try something maybe nobody has tried.

I read it's accuracy is phenomenal. I thought I would gather my own data. So I went outside in the dark and pointed it at the moon.

I couldn't get a reading. Is it broken?

[isddarms]

The green cheese of the moon ablates under laser fire rather than reflecting. You'd probably get a better reading from Mars.

[gsmittle]

Got one of those laser measures as a gift, and have never really used it for anything meaningful. Then the other night, I had this brainstorm to try something maybe nobody has tried.

I read it's accuracy is phenomenal. I thought I would gather my own data. So I went outside in the dark and pointed it at the moon.

I couldn't get a reading. Is it broken?

Probably. Mine broke in a week.

Actually, my understanding is that the laser is for pointing, and the actual distance measure is done with sound waves.

According to the Visual Dictionary of Science (the only resource within easy reach) the moon is roughly 300,000 km distant.

g.

[LCHIEN]

...
So I went outside in the dark and pointed it at the moon.

I couldn't get a reading. Is it broken?

I think you missed (the moon). I know its a big target (2160 miles across), but, If you hit it, you would see a red spot on the moon from the laser. Did you see the red spot?

Besides, the display is too small to show one billion, 218 million feet.

[LCHIEN]

...
Actually, my understanding is that the laser is for pointing, and the actual distance measure is done with sound waves.

...g.

And it's not noted in the instructions for mine,
the laser beam for pointing is very sharp, no dispersion.
The ultrasonic beam for measuring, I would guess has something on the order of 5-10 degrees dispersion.

So even if you can point the laser dot at an object, the measuring beam may be bouncing off an adjacent object that might be closer.

At 20 feet, the laser beam is still a dot, at 20 feet, the sonic beam with 5-degree dispersion would be 1.7 feet wide, with 10 degrees, 3.5 feet wide.

Use them with some degree of skepticism and awareness and you'll be OK.

[cabinetman]

I think you missed (the moon). I know its a big target (2160 miles across), but, If you hit it, you would see a red spot on the moon from the laser. Did you see the red spot?

I really had to squint, but I thought I did see the dot. In my excitement, it was hard to hold the laser steady.
.
.

[scorrpio]

Light takes a bit over a second to travel from Earth to Moon, so Moon is not actually where it appears to be. Depending on current lunar phase, you need to be aiming a bit to the left or a bit to the right.

[gerti]

Light takes a bit over a second to travel from Earth to Moon, so Moon is not actually where it appears to be. Depending on current lunar phase, you need to be aiming a bit to the left or a bit to the right.

But even if you do that right, by the time the light comes back you aren't where you were anymore...

[Habe]

Do a Tim Taylor to it.
Bigger batteries will get you a reading at that distance.

[jwaterdawg]

C'mon guys. Everybody knows that because you are in a non-inertial reference frame (spinning earth) and because of the mass-energy equivalence, E=mc^2, the light beam is curved both on the forward and return trips. Unless you take this into account and target the moon appropriately, you'll never see the spot.

[TheRic]

People, people you are over analyzing!!!! The answer has already been mentioned but no one has noticed yet. The distance is measured by sound, not the laser (Cabman even said he saw the laser point on the moon).


Sound does not travel in a vacuum!!!

[LCHIEN]

People, people you are over analyzing!!!! The answer has already been mentioned but no one has noticed yet. The distance is measured by sound, not the laser (Cabman even said he saw the laser point on the moon).


Sound does not travel in a vacuum!!!

Ric, I think everyone is just having fun...

[Tom Slick]

The reason for no reading is moon cheese is a stealth material and neither sound nor laser will bounce off of cheese back at the direction from which it came.


is it a stabila? those things are amazing, within 1/32" accuracy! it measures with light, I don't know about other brands.

[ragswl4]

As the actual measuring is a function of the sonic wave, based on the speed of sound in air and distance to the moon, it would take approximately 141 days for the sonic wave to travel to and return from the moon.

That would give you enough time to build a REALLY BIG amplifier to boost the returning signal to a level that could be processed by your measuring device.

Sound loss in air is something like POWER of the SIGNAL divided by the square of the distance. P/D^2.


PS: you don't have to stand in the same spot for 141 days, just mark it and return there at the right time. My calculations are not guaranteed to produce results the first time, so you may have to do it more than once.

[LCHIEN]

Rags, I think the battery life is only 139 days. Do you have to leave the unit on?