NOVELTY - The cover (101) has a set of layers (103, 105) e.g. planar sheets, including low emissivity coefficient for radiation e.g. infra-red heat radiation (B), related to radiative heat loss from a body e.g. fluid body (102), with a set of wavelengths in a frequency band. The layers are made of material selected from a group of PVC, ethylene-vinyl-acetate co-polymer and polyethylene. Each layer comprises a thin film made of material selected from a group of metal, metal oxide, PVC and polyvinylfluoride. The film is applied to a base layer made of polyethylene.    USE - Cover i.e. swimming pool cover, for use as a passive solar heating blanket for covering a body e.g. fluid body such as swimming pool (all claimed). Can also be used for covering a sewerage pond in an industrial application, for covering soil to elevate and/or maintain soil temperature to encourage seed growth in early spring or covering a frame to create a shelter for growing plants in a horticultural application, and for agricultural application and commercial application.    ADVANTAGE - The cover reduces radiative heat loss from the body to atmosphere with increased allowance for passive solar heating of the body, so that the cover can act as a passive solar blanket, and a set of optimal conditions for net gain in fluid temperature is provided and compared with an uncovered body of fluid. The cover allows an extension of comfortable swimming season such temperate climate, and reduces electricity consumption required for active heating with reduced carbon dioxide emissions. The cover having optical transparency can allow 77 percent of incident solar radiation of about 200-1100nm to enter the body by transmission through the cover, thus improving or increasing passive solar heating of the body. The cover has a fluid-repellent coating for reducing wetting of a cover surface and limiting ingress of fluid into multiple spaces among the sealed cavities. The layers provide a smooth lower surface for the cover to easily clean the cover, thus reducing available habitat for bacterial growth. The cover provides a consumer with a useful choice for passive solar heating.    DETAILED DESCRIPTION - The cover has a set of sealed cavities filled with gas selected from a group of air, nitrogen, sulfur hexafluoride, argon, xenon or krypton.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a cover in a use condition.    Infra-red heat radiation (B)    Cover (101)    Fluid body (102)    Layers (103, 105)    Sealed air-filled primary bubbles (106)