the Moon not lit by the Sun has served as a proxy. A few direct measurements of the Earth's luminosity have been obtained with the EPOXI spacecraft. While they do not cover much of the phase curve they reveal a rotational light curve caused by the transit of dark oceans and bright land masses across the hemisphere. P. Goode and colleagues at Big Bear Solar Observatory have measured the earthshine and T. Livengood of NASA analyzed the EPOXI data. Earth as seen from Venus near opposition from the Sun would be extremely bright at magnitude −6. To an observer outside the Earth's orbit on Mars our planet would appear most luminous near the time of its greatest elongation from the Sun, at about magnitude −1.5. Mars Only about 50° of the martian phase curve can be observed from Earth because it orbits farther from the Sun than our planet. There is an opposition surge but it is less pronounced than that of Mercury. The rotation of bright and dark surface markings across its disk and variability of its atmospheric state (including its dust storms) superimpose variations on the phase curve. R. Schmude obtained many of the Mars brightness measurements used in a comprehensive phase curve analysis performed by A. Mallama. Because the orbit of Mars is considerably eccentric its brightness at opposition can range from magnitude −3.0 to −1.4. The minimum brightness is about magnitude +1.6 when Mars is on the opposite site of the Sun from the Earth. Rotational variations can elevate or suppress the brightness of Mars by 5% and global dust storms can increase its luminosity by 25%. Gas giants The outermost planets (Jupiter, Saturn, Uranus, and Neptune) are so distant that only small portions of their phase curves near 0° (full phase) can be evaluated from the Earth. That part of the curve is generally fairly flat, like that of Venus, for these cloudy planets. The apparent magnitude of Jupiter ranges from −2.9 to −1.4, Saturn from −0.5 to +1.4, Uranus from +5.3 to +6.0, and Neptune from +7.8 to +8.0. Most of these variations are due to distance. However, the magnitude range for Saturn also depends on its ring system as explained below. The rings of Saturn The brightness of the Saturn system depends on the orientation of its ring system. The rings contribute more to the overall brightness of the system when they are