Context. We present the second Gaia data release, Gaia DR2, consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system. Aims. A summary of the contents of Gaia DR2 is presented, accompanied by a discussion on the differences with respect to Gaia DR1 and an overview of the main limitations which are still present in the survey. Recommendations are made on the responsible use of Gaia DR2 results. Methods. The raw data collected with the Gaia instruments during the first 22 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into this second data release, which represents a major advance with respect to Gaia DR1 in terms of completeness, performance, and richness of the data products. Results. Gaia DR2 contains celestial positions and the apparent brightness in G for approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes and proper motions are in addition available. The sample of sources for which variability information is provided is expanded to 0.5 million stars. This data release contains four new elements: broad-band colour information in the form of the apparent brightness in the GBP (330–680 nm) and GRP (630–1050 nm) bands is available for 1.4 billion sources; median radial velocities for some 7 million sources are presented; for between 77 and 161 million sources estimates are provided of the stellar effective temperature, extinction, reddening, and radius and luminosity; and for a pre-selected list of 14 000 minor planets in the solar system epoch astrometry and photometry are presented. Finally, Gaia DR2 also represents a new materialisation of the celestial reference frame in the optical, the Gaia-CRF2, which is the first optical reference frame based solely on extragalactic sources. There are notable changes in the photometric system and the catalogue source list with respect to Gaia DR1, and we stress the need to consider the two data releases as independent. Conclusions. Gaia DR2 represents a major achievement for the Gaia mission, delivering on the long standing promise to provide parallaxes and proper motions for over 1 billion stars, and representing a first step in the availability of complementary radial velocity and source astrophysical information for a sample of stars in the Gaia survey which covers a very substantial fraction of the volume of our galaxy.

This is the master equation for the evolution of the P representation in the Schrodinger picture. If, as an example, one applies Eq. (17) to the simple caseone has i-which is the well-known equation 4 for the Hamiltonian described by Eq. (18).New radar observations yield a more stringent test of the predicted relativistic increase in echo times of radio signals sent from Earth and reflected from Mercury and Venus. These "extra" delays may be characterized by a parameter X which is unity according to general relativity and 0.93 according to recent predictions based on a scalartensor theory of gravitation. We find that X = 1.02. The formal standard error is 0.02, but because of the possible presence of systematic errors we consider 0.05 to be a more reliable estimate of the uncertainty in the result.General relativity predicts that the round-trip time delay of an electromagnetic wave is influenced by the gravitational potential along the path of the radiation. A test of this prediction involving the transmission of radar signals from Earth to either Mercury or Venus and the detection of the echoes was suggested in 1964.* These echoes are expected on the basis of general relativity to be retarded by solar gravity by an amount 2 A* <=* (4r 0 /c) ln[(r, + r p +R)/(r e + r p -where At, expressed in harmonic coordinates, is the coordinate-time retardation, r 0~ 1.5 km is the gravitational radius of the sun, c is the speed of light far from the sun, r e is the Earthsun distance, r p is the planet-sun distance, and R is the Earth-planet distance. The quantity At is not an observable but is indicative of the magnitude and behavior of the measurable effect as predicted by general relativity. The operational interpretation of the effect has been discussed in detail elsewhere. 3 To test whether or not the echo time-delay data are in agreement with this theory, we may insert an ad hoc multiplicative parameter X on the right side of Eq. (1) and estimate it along with the other unknown parameters that affect the data. 4 This experiment was first performed in 1967 and yielded the result 5 X =0.09 ±0.2 which corre-1132

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