# Practical exercise 2: MEAN SOLAR TIME AND APPARENT SOLAR TIME

## Tools

• sundial,
• watch or mobile phone

## Process

1. The sundial measures the local apparent solar time, which differs from the mean solar time relative to the respective meridian within the time zone. The exercise aims to map the relationship between apparent and mean solar time during a calendar year. First, choose the time that will be as suitable as possible for regular year-round monitoring (so that measurements can be taken at this time throughout the year), for example 3 pm. Remember that this time can only be between 8 am and 4 pm, because in the winter months there is night from about 4 pm to about 8 am. Then measure the exact time (per minute) each day when the sundial shows 3 pm. Record the measured times gradually, preferably in a computer spreadsheet.

2. Calculate the difference between the local time and the time zone given by the longitude of the observation site and indicate it above the table. Then enter the recalculated times corresponding to 3 pm in the sundial in a new column in the table.

3. Draw a graph of the dependence of the difference between apparent solar time and mean solar time. Find out what this difference is called in astronomy. (Answer: time equation)

4. Using the Internet, describe why the difference between apparent and mean time changes during the year. Which two characteristics cause the change? (Answer: the inclination of the Earth's axis and the elliptical trajectory of the Earth around the Sun.)

5. Explain the saying “Lucia shortens the night, but does not add to the day”. Use the terms: apparent solar time, mean solar time, sunset, length of white day.

(Answer: The sun sets on the day of St. Lucia (December 13) at the earliest of the year. If it raised on this day at the latest time as well, it would be the shortest white day. However, the latest sunrise does not occur until the beginning of January. Therefore, the shortest white day is most often on December 20. The fact that the earliest sunset and the latest sunrise do not occur on the same day is caused by the uneven orbit of the Earth around the Sun and the uniform rotation of the Earth around its axis. The saying about St. Lucia is a manifestation of the time equation, i. e. the fluctuation of the length of the time equation during the year. The mentioned saying thus makes the difference between the apparent and mean solar time visible.)