Spatio-temporal characteristics are the crucial conditions for Moon-based Earth observations. In this study, we established a Moon-based Earth observation geometric model by considering the intervisibility condition between a Moon-based platform and observed points on the Earth, which can analyze the spatio-temporal characteristics of the observations of Earth’s hemisphere. Furthermore, a formula for the spherical cap of the Earth visibility region on the Moon is analytically derived. The results show that: (1) the observed Earth spherical cap has a diurnal period and varies with the nadir point. (2) All the annual global observation durations in different years show two lines that almost coincide with the Arctic circle and the Antarctic circle. Regions between the two lines remain stable, but the observation duration of the South pole and North pole changes every 18.6 years. (3) With the increase of the line-of-sight minimum observation elevation angle, the area of an intervisible spherical cap on the lunar surface is obviously decreased, and this cap also varies with the distance between the barycenter of the Earth and the barycenter of the Moon. In general, this study reveals the effects of the elevation angle on the spatio-temporal characteristics and additionally determines the change of area where the Earth’s hemisphere can be observed on the lunar surface; this information can provide support for the accurate calculation of Moon-based Earth hemisphere observation times.

The Earth’s outgoing radiation (EOR) at the top of atmosphere (TOA) is an advantageous goal of Moon-based Earth observations. In this study, we focused on the Earth’s reflected radiation (ERR), which is a component of EOR and has a strong relationship with the Earth–Moon–Sun positions. To determine ERR characteristics, we parameterized the Moon-based Earth observation geometry and proposed a method to simulate the ERR at TOA for a Moon-based sensor under clear-sky and all-sky conditions. The ERR was found to have the distinct cycle of a synodic month and was inversely proportional to the Earth’s phase angle. Clouds increased ERR reflected to space under hemispheric-scale observation; thus, ERR under all-sky conditions was approximately twice as much as that under clear-sky conditions. The ERR was also influenced by incoming solar radiation (ISR), and removal of the effect of ISR on ERR helped reveal ERR characteristics clearly. The results showed that when the Earth’s phase angle was between 0° and 90°, particularly when the Earth’s phase angle was in the range of 30°–60°and 60°–90° under all-sky and clear-sky conditions, respectively, the ERR-ISR ratio could indicate the reflective characteristics of the Earth under hemisphere-scale observations. These results will be helpful in demonstrating Moon-based ERR monitoring.

Earth's outgoing radiation (EOR) at the top-of-atmosphere (TOA) is a vital parameter of earth's energy budget. Moon-based earth observations as one of the scientific goals of the International Lunar Research Station (ILRS) project could provide long-term, well-calibrated, and disk-integrated EOR data to complement existing platforms. To study the difference between a Moon-based platform and existing platforms, in this study, we established a method to simulate outgoing longwave (OLR) and shortwave radiation (OSR) from a Moon-based radiometer. Then, by parameterizing Moon-based earth observations, the relationship between observation geometry and diurnal and seasonal variations of OLR were derived. Finally, the relationship between OSR viewed from a Moon-based sensor and earth's phase is obtained. We summarized three Moon-based EOR observation characteristics. 1) Moon-based observational scope is almost earth's hemisphere, and Earth–Moon distance has great effects on EOR. With the Moon moves from perigee to apogee, the OLR decreases almost 40% and OSR decreases about 60%. 2) Diurnal and seasonal variations also influent OLR, and compared correlation coefficients between OLR removing the effects of seasonal with diurnal, the former has bigger value, meaning hemisphere-scale observations is less sensitive to seasonal signals. 3) A Moon-based platform can observe the earth with different earth's phases. For OLR, earth's phase changes earth's scene. For OSR, with the increase of earth's phase, OSR is greatly decreased, the ratio of incoming shortwave radiation (ISR) is close to earth albedo when the earth's phase is 0°. Our results offer new understanding that will help to facilitate Moon-based monitoring of EOR.

Observation angles are of great importance with respect to Earth observation platforms. The richness of angular combination, i.e., the combination of three observational angles (viewing zenith angle, solar zenith angle, and relative azimuth angle), is an important parameter to illustrate the angle sampling capability of an Earth observation platform. Here, the angular combination characteristics of a Moon-based platform were investigated and compared with existing artificial satellites. Furthermore, the effects of the time sampling interval on the angular combination characteristics were analyzed using a newly established angular combination number index (ACNI). Results show that a Moon-based platform can complement angular sampling observations from existing satellites. We found that the time sampling interval has different effects on the angular combination for different observed points. Accordingly, the Earth’s surface can be divided into two zones with respect to its sensitivity to the time sampling interval. When the time sampling interval increased from 10 min to 2 h, the maximum loss of the angular combination reached 50% for the observed points in the mid–low latitude zone.