Introduction

	*Introduction*

	Earthshine is is sunlight reflected by the Earth which is visible as a dim image of dark portion of the lunar disk. The intensity of earthshine relates to the average of the Earth albedo, so it relates to the global temperature of the earth. A fraction (albedo, about 30%) of sunlight incident on the Earth is reflected by the earth atmosphere and surface back into space. The rest of solar energy is absorbed by the Earth (atmosphere and surface) and converted into heat. This energy re-emits into space in the infrared range as an approximate black-body radiation of 225°K (The spectrum of this re-emitted energy is well separated from the incident sunlight whose effect temperature is about 5780°K). Thus the albedo relates to the global temperature of the earth. The change in albedo relates to the change in the global temperature T. More than seventy years ago, Danjon first tried to determine the average earth albedo from earthshine measurements (Danjon 1928, 1936, 1954). He used the ratio of the intensity of the dark portion to the intensity of the bright portion to reduce the effects of the atmosphere and solar intensity. However, the error bar of his measurements, about 5%, is too large to determine the variation of global temperature. Recently the earthshine measurements at the Big Bear Solar Observatory show that the error bar of albedo can be as small as 1%, which corresponds to about 0.25% change in global temperature (Goode et al. 2001). Thus earthshine measurements with modern technology becomes a promising method to measure the variation of the global temperature. Since earthshine measurements at one site can yield the albedo averaged over only part of the Earth. Our ultimate goal is to to build a ground-based global network to measure the long-term variation of earthshine to learn the long-term variation of the global temperature. To make long-term observations feasible with a low cost, we need to use automated telescopes. The goal of our design is to make the telescopes operate automatically without human care for a reasonable period, for example, a couple of weeks. The first step of the project is to build a prototype automated telescope at Lulin Mountain. Then we will build a few more telescopes to install at suitable sites around the world. The design of the prototype telescope is briefly described as follows. A 3.5-inch ruggedized Questar telescope (Maksutov type) and a German-type equatorial mount are used. A 10^-5 neutral density filter is used to reduce the intensity of the bright portion of the lunar disk such that the dark portion and the bright portion can be measured simultaneously. A heat-block filter will be also used to remove the near-infrared energy. A 16-bit 1024 x 1024 air-cooled CCD will be used to take images. Tracking the Moon is more difficult than tracking stars and the Sun because the shape of the dominant bright portion of the lunar disk changes with time. We will use both the passive tracking (using the pre-determined coordinates) and the active tracking (using the observed lunar images to adjust pointing). Pointing of the telescope will be measured with an optical decoder whose accuracy is arcsecond. To make the telescope robust, we minimize electronic hardware by using a digital signal processor (DSP) to control the telescope. This allow us to improve or update the control system by modifying the software of the DSP. The DSP will be controlled by a Linux-based computer. A GPS will be used to provide accurate time.