In trading on Monday, shares of the Guggenheim Solar ETF ( TAN) entered into oversold territory, changing hands as low as $39.581 per share. We define oversold territory using the Relative Strength Index, or RSI, which is a technical analysis indicator used to measure momentum on a scale of zero to 100. A stock is considered to be oversold if the RSI reading falls below 30.In the case of Guggenheim Solar, the RSI reading has hit 26.5 — by comparison, the RSI reading for the S&P 500 is currently 43.9. A bullish investor could look at TAN's 26.5 reading as a sign that the recent heavy selling is in the process of exhausting itself, and begin to look for entry point opportunities on the buy side. Looking at a chart of one year performance (below), TAN's low point in its 52 week range is $31.77 per share, with $50.00 as the 52 week high point — that compares with a last trade of $39.90. Guggenheim Solar shares are currently trading down about 3% on the day.