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# S17.4.3

Knows that a solution and its consequences must be tested against the needs or criteria the solution was designed to meet.

### Seeing Interference Fringes with a Telescope

Astronomers build larger telescopes not only to collect more energy but also to resolve finer detail in the objects to be studied. While groundbased optical telescopes with "filled" apertures of up to 10 m have been built, and 30 m and larger-aperture designs are being studied, another method of observations permits equivalent apertures spanning tens to hundreds of meters to be used. Radio astronomers have used these instruments, called interferometers, for decades, with equivalent apertures spanning thousands of kilometers.

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### Making Months

It is easy to take our calendar, with its system of weeks and months, for granted. But this system is really just a societal choice -- it has been debated and modified in the past. It will also have to be re-examined when humans colonize other planets. 'Making Months' illustrates a use of factoring, in a decision-making process.

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### How Many Days Are in a Year?

Background: The true length of a year on Earth is 365.2422 days, or about 365.25 days. We keep our calendar in sync with the seasons by having most years 365 days long but making just under 1/4 of all years 366-day "leap" years. Exercise: Design a reasonable calendar for an imaginary planet. Your calendar will consist of a pattern of 366-day "leap" years and 365-day regular years that approximates your planet's average number of days per year.

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