1. The problem asks to determine the temperature shown on the thermometer. 2. The thermometer scale is marked at 0, 10, 20, and 30 degrees Celsius. 3. The red mercury column rises slightly above the 20 degrees mark, close to 23 degrees Celsius. 4. Since the mercury level is between 20 and 30 degrees, and closer to 20, we estimate the temperature as 23 degrees Celsius. 5. Therefore, the temperature shown on the thermometer is $23^\circ C$.