National Solar Heating and Cooling Information Center Hotline: 800/523-2929 (Monday through Friday, 9 a.m. to 5 p.m.). Mailing address: Solar Information, Dept. M. Box 1607, Rockville, Md. 20850. Solar Power Activities. By Eleanor Villapando and Beverly S. McNamara. $8.95. Available from Resources for the Gifted, 3421 N. 44th St., Phoenix, Ariz. 85018. 602/959-9770.

These days there are hotlines are everything under the sun. And now there's even one for the sun.The National Solar Heating and Cooling Information Center, established by the Departments of Energy and Housing and Urban Development, is in business to educate people about solar energy. Truly a hotline, the center's toll-free phone number attracts more than 3,000 calls a week. If the staff can't give a quick answer to your solar query over the phone, they'll research it and write you back. Nothing is too technical, or simple, for them. The center also receives more than 1,200 written requests weekly.

Another contribution to solar education is aimed at the young set. Solar Power Activities is a workbook for children 8 to 16 that puts their curiosity and enrgy to work on 32 solar-energy projects that include concocting a solar hot dog cooker, a solar clock, a solar water heater, and four kinds of solar water stills, all using simple household materials. Each activity is accompanied by drawings and quizzes on what's been learned.