2018年IAO理论低年组第2题-火星大冲

英文题目

Great oppositions of Mars. On the morning of July 27, 2018, Mars was in a great oposition (see attached ephemeris), The great oppositons of Mars (the situations when this planet is seen most brightly in comparison with the visibility during other oppositions) are repeated every 15 or 17 years. So the previous opposition (which is ales called the "greatest", since Marswas so bright only once in many centuries) was on August 28, 2003, and the next one will occur in 2035.

2.1. Determine the constellation, in which Mars was during this year opposition.

2.2. Considering the orbits of the Earth and Mars as circular, and based on the date of opposition 2003, calculate the dates the oppositions in 2018 and 2035 would occur.

2.3 Explain, why the difference appeared between the calculated and real dates of the opposition 2018. (Draw a figure that clearly demonstrates your explanation.)

2.4. In which of the oppositions, 2018 or 2035 is Mars brightest? (The answer must be justified by drawing a figure).

中文题目

火星大冲 2018年7月27日上午，火星到达大冲位置（见附录星历表【注：暂无】）。火星大冲（大冲时行星的视亮度在所有冲日事件中是最亮的）每15或17年发生一次。所以上一次大冲（也被称为“最”大冲，因为这一次的火星是许多世纪以来最亮的）发生在2003年8月28日，下一次大冲将发生在2035年。

2.1. 写出今年火星大冲时，火星位于哪个星座。

2.2. 假设地球和火星的轨道为圆轨道，基于题目中给出的2003年火星大冲的发生日期，计算2018年和2035年发生火星大冲的日期。

2.3. 试解释计算得到的2018年火星大冲日期与实际日期间存在差别的原因。（请画草图说明）

2.4. 2018年与2035年的火星大冲，哪一次更亮？（请画图证明你的答案）

解答

英文解答

2.1. During the opposition, the planet is at a point approximately (up to an angular distance form the ecliptic at this moment) opposite to the Sun, that is, located where the Sun was six monthe ago. The Sun is in the constellation Campricorn on January 27.

2.2. The oppositions are repeatde every synodic period. The eyniduc of Mars is:

$$T_S = 1/(1/T_E - 1/T_M),$$

$$T_S = 1/(1/365.256 - 1/686.98)\quad 779.93\ days,$$

Approximately 15 years passed from 2003 to 2018, and there are 15·T_E / T_S ≈ 7.02 synodic periods of Mars during this period. So, the opposition 2018 was to occur after 7 synodic periods earlier on August 28.

The easiest way to calculate the date is as follows. For 7 synodic periods of Mars 7×779.93 ≈ 5459 days pass. From August 28 2003, to Augusr 28, 2018, 365×15+4 = 5479 days pass. Thus, it turns out that in 2035 the opposition of Mars should be 11 days later than ugust 28, that is, Septmeber 8.

2.3. The difference appears from face that in our calculations we take the avrage angular velocitu of Mars in orbut, and iin perious close to great oppositions, Mars is near the perihelion of its orbit, and its speed is larger. As a result, the difference in the angular velocities of the Earth and Mars is smaller. Therefore, to change the positoin of Mars relative to the Earth takes more time. That is why, the farther the opposition from the perihelion point of Mars orbit (which is at the ecliptic longitude that the Earth passes aroune August 28), the greater the error. Calculations inthe approximation of circular orbits give about 1.5 times the underestimated interval between the dae of the calculated opposition and the date on which the "greatest oppositions" take place (that is, August 28).

2.4. The position of Mars during the opposition of 2035 is closer to the positiom of the perihelion of the orbit than in 2018, therefore in the opposition of 2035 Mars will be brighter.