From what I understand, suntanning is not permitted on Shabbos, but sitting or walking in the sun is not forbidden. May I sit in the backyard on Shabbos even though i will get tanned? What exactly makes it assur? If I take off my shirt, does it then become assur? Whereas if I sit with a shirt, it is not assur?
Purposely suntanning on Shabbos is controversial if it is permitted or not. The poskim that forbid it agree that it is permitted to walk or sit in the sun if the intention is not to tan the skin. They only prohibit it if the intention is to suntan, whether you have your shirt on or not. Taking your shirt off though will show that your intention is to tan.
Machzei Eliyahu 65:24, Zocher V’Shomer page 11, Chelkes Yaakov 4:17, Oz Nedberu 2:30, Shemiras Shabbos K’hilchoso 18:footnote 70, Migdal Dovid 3:page 552, Klalim page 353, KItzur HIlchos Shabbos 21-4, Emes L’yackov ftnt. 377, NIshmas Shabbos 327-215, Hilchos Shabbos B’shabbos 59-ftnt. 14, R’ Heinenman shlit”a is quoted (Kashrus Kurrents 30-1 pg.6 as qouting R’ Moshe that it is permitted to sit in the sun even though it will cause the person a tan, (he doesn’t say that it is permitted to purposely do so.).