"Yes, but it will be colder to-night," said Jack. For they had landed
on the moon in bright sunlight.

"To-night?" questioned the German scientist, with a smile.

"Yes, it's always colder when the sun goes down," went on the lad.

"You have forgotten one thing," said Mr. Henderson, with a smile at his
young protege. "You must remember, Jack, that the nights and days here
are each fourteen days long--that is, fourteen of our days."

"How's that?" asked Jack.

"Why," broke in Mark, who was a trifle better student than was his
chum, "don't you remember that the moon rotates on its axis once a
month, or in about twenty-eight days, to be exact, and so half of that
time is day and half night, just as on our earth, when it revolves on
its axis in twenty-four hours, half the time is day and half the time
is night."

"Sure, I ought to have remembered," declared Jack.

"Mark is right," added Mr. Henderson. "And, as we have most fortunately
arrived on the moon at the beginning of the long day, we will have
fourteen days of sunshine, during which we may expect the temperature
to remain at about twenty-eight degrees. But now about the atmosphere."
"We will test that directly," went on the German. "It will take some
time longer, though."