Opposition effect of the Moon Yu. Velikodsky et al. ELS’XII 302 Opposition effect of the Moon from ground-based and space observations Yu. I. Velikodsky *,1 , V. G. Kaydash 1 , Yu. G. Shkuratov 1 , N. V. Opanasenko 1 , V. V. Korokhin 1 , and G. Videen 2 1 Institute of Astronomy, Kharkiv National University, Sumskaya 35, Kharkiv, 61022, Ukraine. 2 Space Science Institute, Silver Spring Maryland 20905 USA. We study phase ratio images of the lunar nearside at low phase angles (1.6° and 2.7°) using ground-based telescope observations. The ratio appears highest for highlands of intermediate albedo, while bright craters have lower values. An average phase curve obtained with ground- based and spacecraft photometry of the Moon at phase angles 0.25–73° is presented. INTRODUCTION The brightness opposition effect (BOE) is a rapid increase of surface brightness seen when the phase angle  approaches zero. There are two mechanisms governing the BOE of plane- tary regoliths: the shadow-hiding effect and coherent backscattering enhancement [e.g. 1,2]. The relative contribution of each mechanism depends on , albedo, and surface structure. Coherent backscattering can be significant only if the surface is rather bright. The BOE is observed for many atmosphereless celestial bodies including the Moon at  below 5–10° [1]. There are so far discrepant data about the role of coherent backscattering in the formation of the lunar BOE. Low-phase-angle observations of the Moon are difficult. Absolute photome- try has low accuracy [3], spacecraft data are scarce, and ground-based observations cannot be carried out at <1° because of the lunar eclipse. Relative lunar photometry can be performed by dividing one image obtained at 1 by another image acquired at a generally larger  2 . This method can be very effective and was applied to select Clementine [4] and SMART-1 [5] data. Similar phase ratio technique was applied to Earth-based telescope data [6,7]. We use this method to produce a phase ratio (1.6°/2.7°) image of the lunar disk. OBSERVATIONS During a two-month campaign in 2006, we carried out quasi-simultaneous imaging photome- ty of the Moon and the Sun using a 15-cm refractor at Maidanak Observatory (Uzbekistan) [3,8]. We have presented results of absolute photometry [3], in particular, phase dependencies at  = 1.6 … 73° for several lunar areas acquired at 603 nm. We also obtained a series of lunar images near opposition suitable for phase-ratio analysis. The series covers the range of  = 1.6 … 3.1° (hereafter,  values correspond to the lunar disk centre). During the observa- tions near opposition the photometric equator, the direction from the sub-observer point to * Corresponding author: Yuri I. Velikodsky (dslpp@astron.kharkov.ua)