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This work presents the development and construction of the Heliometer of theObservat\'orio Nacional/MCTI. This instrument is designed to monitor changes onthe solar diameter with the accuracy of the next-generation solar satellites. Areview of the heliometric method is made and the building and testing of 4prototypes is described. The instrument has a mirror objective split indihedral, formed by the hemi-sections of a parabolic mirror. The materials thatform the instrument have thermal and mechanical stability to 10^(-7). Thenumber of optical parts is minimized and their quality is greater than{\lambda}/12. An original software for the automated collection and analysis ofthe images was developed. With its latest version fully developed, we conductedan observational campaign of 9 days, deriving more than 70000 heliometricimages of the Sun. The measured solar diameter has a standard deviation of 0.5arcseconds, with no instrumental bias, and limited only by the provisionalatmospheric modeling. Therefore, by improving the statistics, an accuracy atmilliarcseconds can be achieved. A study of the correlation between the solardiameter and the geomagnetic field intensity was made. A correspondence betweenthe peaks of the series related to solar activity, namely, the semidiametervariation, the flares index, and the sunspots counts, and the number ofnegative peaks on the intensity of the geomagnetic field is found. It must becautioned though that it is a complex correspondence, requiring different modesof response and phase dependence on the stage of the cycle of solar activity.The straightforward interpretation of observational evidence indicates that thesemidiameter of the Sun seems to vary significantly prior to the correspondingvariations of the geomagnetic field. The effect is most evident in the downwardphase of the solar cycle.

The development of the Heliometer of the Observatorio Nacional of Rio de Janeiro and application to the study of the Sun-Earth system