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The solar constant is the density of the Sun's flux (amount of EMR energy per unit of an area perpendicular to the direction of the EMR) when the flux approaches Earth, or precisely, after traveling 1 AU. The current typically-cited value is 1.361 kilowatts per meter², an average derived from measurements over roughly three decades. Despite the word constant, the actual flux reaching us is not: because Earth's orbit is a bit eccentric, the flux actually hitting Earth atmosphere varies around this solar constant, currently varying roughly 7% over the course of each year. Also, it is more at solar maximum.