SB 5.24.3

tan niśamyobhayatrāpi bhagavatā rakṣaṇāya prayuktaṁ sudarśanaṁ nāma bhāgavataṁ dayitam astraṁ tat tejasā durviṣahaṁ muhuḥ parivartamānam abhyavasthito muhūrtam udvijamānaś cakita-hṛdaya ārād eva nivartate tad uparāgam iti vadanti lokāḥ.
Word for word: 
tat — that situation; niśamya — hearing; ubhayatra — around both the sun and moon; api — indeed; bhagavatā — by the Supreme Personality of Godhead; rakṣaṇāya — for their protection; prayuktam — engaged; sudarśanam — the wheel of Kṛṣṇa; nāma — named; bhāgavatam — the most confidential devotee; dayitam — the most favorite; astram — weapon; tat — that; tejasā — by its effulgence; durviṣaham — unbearable heat; muhuḥ — repeatedly; parivartamānam — moving around the sun and moon; abhyavasthitaḥ — situated; muhūrtam — for a muhūrta (forty-eight minutes); udvijamānaḥ — whose mind was full of anxieties; cakita — frightened; hṛdayaḥ — the core of whose heart; ārāt — to a distant place; eva — certainly; nivartate — flees; tat — that situation; uparāgam — an eclipse; iti — thus; vadanti — they say; lokāḥ — the people.
Translation: 
After hearing from the sun and moon demigods about Rāhu’s attack, the Supreme Personality of Godhead, Viṣṇu, engages His disc, known as the Sudarśana cakra, to protect them. The Sudarśana cakra is the Lord’s most beloved devotee and is favored by the Lord. The intense heat of its effulgence, meant for killing non-Vaiṣṇavas, is unbearable to Rāhu, and he therefore flees in fear of it. During the time Rāhu disturbs the sun or moon, there occurs what people commonly know as an eclipse.
Purport: 

The Supreme Personality of Godhead, Viṣṇu, is always the protector of His devotees, who are also known as demigods. The controlling demigods are most obedient to Lord Viṣṇu, although they also want material sense enjoyment, and that is why they are called demigods, or almost godly. Although Rāhu attempts to attack both the sun and the moon, they are protected by Lord Viṣṇu. Being very afraid of Lord Viṣṇu’s cakra, Rāhu cannot stay in front of the sun or moon for more than a muhūrta (forty-eight minutes). The phenomenon that occurs when Rāhu blocks the light of the sun or moon is called an eclipse. The attempt of the scientists of this earth to go to the moon is as demoniac as Rāhu’s attack. Of course. their attempts will be failures because no one can enter the moon or sun so easily. Like the attack of Rāhu, such attempts will certainly be failures.