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Artificial Atmosphere

I was initiated to this research by my advisor Dr. Jack 0. Burns at a time when I was still looking for a good Ph.D project. It has been a rewarding experience since it allowed me to develop some computational skills. The most important point about the Moon is its low atmospheric density and its surface seismic stability which make it an ideal environment from which to conduct very high resolution astronomical imaging and activities such as high-vacuum materials processing.

An important question to answer is whether the gas and dust injected into the lunar atmosphere by exploration and colonization could overwhelm its removal (by thermal evaporation and solar wind interaction) and lead to a significant contamination of the lunar environment. The task was to study the dispersal of gases arising from operations on a future lunar outpost and to predict its effects on astronomical observations from the Moon. I modeled analytically the dispersal of gases into the lunar atmosphere using continuous (e.g., mining and habitation venting) and impulsive (e.g., rocket exhaust) injection mechanisms and assuming a collisionless, isothermal atmosphere. In the impulsive injection case, I have found that the neutral atmosphere and associated ionosphere both decay on time scales of about 20 minutes. In the continuous injection scenario, the atmosphere near the outpost grows and reaches a steady state after approximately 20 minutes. For a moderate injection rate (1 kg/s) any significant atmosphere is limited to within 1 km of the source. The resulting ionosphere impacts radio astronomical observations only within 10 km of the source. Both direct transport and diffusive transport (i.e., repeated bounces off of the lunar surface) were considered in the code. It is concluded that at these injection rates and within the constraints of our assumptions, an artificial lunar atmosphere is not a serious detriment to astronomical observations and high-vacuum materials processing.

Crescent Visibility

The Islamic calendar is principally a lunar calendar. The determination of the first day of any Islamic month is not a simple matter, but rather a complex one. The question is how soon after the new Moon can we spot the lunar crescent in the evening twilight? Several civilizations before us faced the same question, and several criteria were introduced for a possible sighting. The most common one is that the thin lunar crescent should be at least one day old at the time of sunset. Each succeeding day the Moon sets later, increasing the chance that it will be seen. Sightings of the Moon within 20 hours of its new phase are extremely rare. However, some records have been set such as the naked-eye visibility of a 15.4 hours crescent in 1871, a 14.9 hours visibility in 1972, and a 13.5 hours visibility in 1988.

The sighting of the thin lunar crescent depends factors such as the atmospheric clarity, the sky brightness, and the sensitivity of the eye during the evening twilight. The uncertainty in the weather adds more problems in the exact determination of the Islamic months. One general method used by earlier Muslim astronomers for possible sighting is the time lag between sunset and moonset. The thin crescent can be visible if the Moon sets at least 48 minutes after sunset. This limit, however, can be either shorter or longer depending on the location and the season. Another equivalent method is that the angle of separation of the Moon and the Sun should be at least 12 degrees at sunset.

Several algorithms exist today to predict the first visibility of the crescent. Some of these algorithms use just the old criteria methods described above without taking into account the observing conditions. Other algorithms, however, include the brightness of the sky, the atmosphere of the observing site and even the characteristics of the human eye. My work falls in this last domain. I am working on an algorithm that allows me to predict where on the Earth the crescent can be first sighted. The problem is to narrow the marginal error of such a prediction. An international lunar dateline can be worked out after every new Moon. Observers west of this line are those who have most chances to see the crescent on the first night, while observers to the east should wait one more day. This is due to the line which marks the sunset since it sweeps westward as the Earth rotates.

The accuracy of such an algorithm is based upon a good reliable sightings of the crescent. I am presently collecting data on such sightings in order to have an accurate code. Hopefully, it will be good enough to help us predict the first visibility of the crescent. In any case, the algorithm is set to help us, not to determine automatically the beginning of any Islamic month.

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