Given the consequences of climate change on our lives, we need to be able to predict how solar activity is likely to vary several decades ahead. With the level of greenhouse gases in the atmosphere currently rising, it will be useful to know whether this activity is going to wax or wane in a few decades’ time. Modelling solar activity is one possibility. At present, we have the capability to forecast solar activity at best out to a range of a few years, using an empirical law based on measuring the intensity of the Sun’s magnetic field at the end of the 11-year cycle indicating the amplitude of the next cycle. We can also attempt to deduce the secular trend by comparing the value of the solar constant measured for two successive solar minima. Radiometers are very precise instruments (10-3), but still not precise enough for this task. Measurements from two radiometers in different orbits may differ by as much as 1 W/m² or more, which is why readings have until now been acquired continuously by at least two radiometers observing simultaneously and by overlapping missions. Our knowledge of these instruments has enabled measurements to be placed on a single scale. Fröhlich (2003), Fröhlich and Lean (2002) and Dewitte et al. (2005) give equal values of the solar constant in the vicinity of solar minima, within the margins of uncertainty, which suggest there is no secular trend. On the other hand, Willson and Mordvinov (2004) show a rise in activity of 0.04% per decade. This difficulty stems from the fact that the variability being sought is smaller than the instruments’ absolute precision. Another technique observed the number of sunspots over time to extract the principal modes (frequency and amplitude). An extrapolation is then possible (Damon and Jirikowic, 1992), but the degree of uncertainty depends on the uncertainties in the entire dataset, particularly older data.
Picard proceeded in a different fashion, using a geometric measurement and a reference that future space-based instruments will be able to use. The idea is to match the Sun’s diameter against angular distances of stars with positions 10 to 20 arcminutes apart. Fifty doublets have already been found. However, the proper motions of stars already measured by Hipparcos will be taken into account and re-measured by astrometry missions like Gaia.