"Our knowledge should be through the sastra. That is practical. And our process of knowledge is this: we get knowledge from sastra, like Bhagavad-gita, Srimad-Bhagavatam, Vedic knowledge. We do not claim to be very big scientists. That is not possible. But we get knowledge from the best scientific man or person, the Personality of Godhead, Krsna, Vyasadeva, Narada, Asita, Devala, later on the acaryas, Ramanujacarya, Madhvacarya, Sankaracarya, Caitanya.
"Our process of knowledge is not any speculation: "It may be," "Perhaps." No. We don't accept this knowledge. "It may be," "Perhaps" -- these are all foolishness. That means one who has no perfect knowledge, he will say, "It may be," "Perhaps." One who has definite knowledge, why he will say, "It may be"? It must be.
"So far as we get information from Srimad-Bhagavatam, there is sun, then moon, according to Bhagavatam. We were talking about these things: whether the moon is the first planet or the sun is the first planet. So far as we understand from Vedic literature, the moon is the second planet. The sun is the first planet.
"If we consider like that, then the moon is beyond the sun planet. The estimation is there in the Srimad-Bhagavatam:1,600,000 miles above the sun, the moon is situated. Now, if we take consideration of the sun planet situated 93,000,000 miles from earth, then add 1,600,000 miles again, it comes to 95,000,000 miles away from the earthly planet. And how you can reach there in four days, 95,000,000 miles away? If we apply our common sense, then it appears they have never gone to the moon planet. It is all bogus propaganda. It is not possible."
(Srila Prabhupada - Lecture, Nov. 28, 1975)