Observations of the Moon


Joachim Köppen Strasbourg 2011



The Moon is a difficult but very interesting object: It is a rather faint source, usually 0.5 dB above the level of the empty sky, which is comparable to the normal fluctuations of the background noise. Its emission is the thermal radiation from its surface. Since the surface is heated by the solar radiation, the temperature depends on the lunar phase. The fascinating thing is that the average temperature is not highest at Full Moon, when all the surface is lit, but several days later. This was discovered in the 1950s and indicated that the surface was not solid rock, but covered with a layer of dust, which takes some time to heat up under sunlight. This finding caused quite some concern for the program to land men on the Moon, as it was not clear how thick this layer was, and whether it would be able to support a landing vehicle ... These temperature variations are only about 10 percent, which presents quite a challenge in view of the faint signal and the difficulties to obtain sufficiently good quality observational material. But what would we do without challenges?

Observational procedure

Important notes:

After starting up the system, described here, we recommend to follow this sequence:


Analysis

Let's suppose that we use Microsoft Excel to do the interpretation of the data. Then we recommend to follow this sequence:

To determine the temperature on the surface of the Moon, we also need to know the fraction with which the Moon's disk fills the antenna beam:

Finally, it is worth estimating the errors on the temperature. What are the sources of error:

Which error is the dominant one? Can we improve our technique, and where?


More about our results

Our first successful observation, after the refocussing the the telescope, was done on 26 April 2007. We got full scans like shown below, which was taken five days before Full Moon:

The signal is only 0.5 dB over the background level, but with some practice, one can locate the moon ... The observations shown also display the flux calibrations on the Holiday Inn hotel; if we assume that it has a temperature of 300 K, the antenna temperature for the moon was 15 K; from the scan profile, one deduces an antenna HPBW of 1.9 (which is larger than measured with the much better profile obtained with the Sun - obviously an effect of the greater imporatnce of noise), and it gives a temperature averaged over the lunar surface of 211 K! A second scan taken one hour later gave the same numbers. The small wiggle just before 18:45 was the manual search about the expected position for the lunar signal. Once found, we placed the telescope a bit to the west and a bit higher, and waited for the moon to pass through the beam ... and sometimes it does so!


However, the Moon is a very difficult customer, as Zahrah Musa found out during many nights in 2008: Not only rain clouds make it impossible to pick up the weak lunar signal, and the thermal emission from the clouds themselves can be stronger than that of the moon. Also the slightest veil of clouds can upset the signal by its emission. So when you try to locate the Moon, and find some enhancement of the signal, you may also find that this signal fades away as slowly as the Moon would do it ... since clouds also move across the sky ... but then the signal may go up again ... eventually you get really confused and frustrated.

The plot below (from Marina Lemberg's data) shows the signals during a day when sun, rain, and winds came and went in succession: The high signal at lunchtime is the transit of the Sun, but the other peaks are simply rain clouds passing over:

It is clear that against these clouds, the Moon - at about +42.5 dBV on that day - would have little chance to be observable!

On the next day - sunny and blue skies - Marina had the chance observation of the New Moon following the Sun in its lunchtime transit:

The dark blue curve is the lunar signal multiplied by ten, for better visibility. The green curve is a smothed version of the blue curve. Both show that the Moon has an antenna temperature of about 20 K, which is - within the error bars - the same as we had observed at full moon. To further improve our techniques of observation and analysis were our next steps ...


We thus learned that a major limitation is not due to our instrument ... it is the natural weather which we can do nothing about! The Moon remains a nice and challenging target, but you need a cloudless sky for that! ..... But it ain't necessarily so!!!!! The following observations were done when the Full Moon was behind a layer of fog. It was still fairly easy to distinguish the dark mare and bright highlands on its face, but I would not have bothered to bring out an optical telescope to view the Moon. However, the radio data are as good as those taken during a cloudless evening a few days earlier!

More details of our results and techniques may be found here:


Results from 2011

Employing this improved technique possible with the new software, Mary-Anne Fobert was able to take 106 individual measurements of the Moon, covering the entire lunar phase range. To our great surprise, meaningful measurements were obtained during nearly all weather conditions that were present this winter: clear sky, mist, fog, and overcast sky. Only during a snowfall the lunar signal could not be pickd up. A comparison of her data with Monstein's curve (in red) is shown below:

The average values correspond well with these other observations, however the scatter is still too large to detect the systematic relation with lunar phase. Some data - like the one with a very low value of 50 K - obviously come from observations where the Moon did not pass through the center of the antenna lobe. A careful look reveals that the readings taken when the Moon was observed at elevations less than 20 degrees (orange dots) are systematically higher than the ones taken at higher elevations (blue diamonds). The reason is quite simple: at lower elevation we observe through a larger airmass and hence we receive a larger amount of thermal radiation from the atmosphere than if the Moon is observed closer to the zenith (see also here). Since in our analysis we do not yet separate the elevation-dependent atmospheric noise from the constant receiver noise, as it would require numerous additional measurements, we overestimate the lunar temperatures!

An even closer look shows that the weather does not affect the measurements

Blue diamonds indicate clear skies, green squares overcast skies, fog, or mist, but all taken at elevations higher than 20 degrees. Yellow symbols refer to observations at low elevation, the circles showing clear sky conditions. It is clearly the elevation effect that dominates!

Lastly, we had become aware that our previous elevation for the flux calibration was not sufficiently low enough for the Holiday Inn building to fill the antenna beam completely. Hence, we now use point 2 degrees lower. As a consequence, the system temperatures (and hence the derived lunar temperature) now are significantly lower, as shown below:

Yellow dots are measurements done with the old calibrator position, red dots with our new position. It demonstrates how our system temperature increases due to the higher sky noise at low elevations, but that the measurements with the old calibrator give values systematically higher by about 50K.

Having identified these two effects, we now can refine our observational techniques! Next round ...


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last update: 24 Mar. 2011 J.Köppen