# Hubble Expansion: Moon Orbital Distance & Error Check

• John Richard
In summary, the conversation is about the distance to the moon and how it is calculated by NASA. There is a question about whether or not it is valid to apply the Hubble expansion to the distance between Earth and the moon, and if there is enough error in the measurement to account for the difference between the Hubble and NASA calculations. It is mentioned that the moon is moving away due to orbital dynamics such as tides.
John Richard
Hello,
A couple of questions:
The distance to the moon is calculated as increasing by 38 millimetres per year by the laser ranging work of NASA following Apollo 11.

When I apply the Hubble expansion coeficient to the centre to centre distance from Earth to the Moon, I get just over 28 millimetres per year relative to todays measured distance.

Firstly, is it valid to apply the Hubble constant to relatively close orbitals in this manner?

Secondly, if it is valid, is there sufficient error in the centre to cenre distance being stated as an average of 385,000Km to compensate for the difference, (38 to 28)?

I am of course checking my maths.

Thanks for any help

John

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No, it isn't valid to apply the Hubble expansion on these scales, it's not that the expansion term is small but because the Hubble result applies to an idealized homogeneous Universe as therefore does not work for bound structures that are thousands of times more dense than the mean density.

I remember learning once why the Moon is moving away, I think it's to do with some orbital dynamics such as tides, but I'm hazy. Hopefully someone else can help you on that part.

Thanks Wallace.

I had a search around the net and found some info confirming what you said Wallace, it is tides, thanks for the clue.
John

## 1. What is Hubble Expansion?

Hubble expansion is the observed phenomenon where the universe is expanding at an accelerating rate.

## 2. How does Hubble Expansion affect the Moon's orbital distance?

Hubble Expansion does not directly affect the Moon's orbital distance. However, it does have an indirect effect on the Moon's orbital distance due to the overall expansion of the universe, which may cause the Moon's orbit to gradually increase over time.

## 3. What is the current orbital distance of the Moon?

The current average orbital distance of the Moon is approximately 384,400 kilometers from Earth.

## 4. How is the Moon's orbital distance measured?

The Moon's orbital distance can be measured using various methods, including radar, laser ranging, and lunar laser retroreflector experiments.

## 5. How do scientists account for errors in measuring the Moon's orbital distance?

Scientists use multiple methods of measurement and compare the results to ensure accuracy. They also take into account factors such as the Moon's elliptical orbit and gravitational interactions with other celestial bodies.

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