There is no rising and setting of the sun in space so that an astronaut can find out the exact period to keep the Sabbath. This could pose a dilemma for some space travelers. But an astronaut can keep the Sabbath at the time he was keeping it at home (wherever that may be) just as he did before he left Earth.
For that he would have to use some kind of earth-time standard and dedicate time for God’s worship. This way he can keep the Sabbath according to the Word of God as best as he knows how, and the Lord will accept his best service.
God looks at the heart of man (Psalms 139:1) and if He sees that man is willing to obey and does his best towards that end, the Lord receives his love and worship. “Then hear in heaven Your dwelling place, and forgive and act and render to each according to all his ways, whose heart You know, for You alone know the hearts of all the sons of men” (1 Kings 8:39).
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In His service,