Which scriptures describe the chariot of Sun God?
It is described in Srimad Bhagavat Purana.
Srimad Bhagavat Purana Canto 5, Chapter 21 describes
The movement of Sun and it's Chariot.
यस्यैकं चक्रं द्वादशारं षण्नेमि त्रिणाभि संवत्सरात्मकं समामनन्ति तस्याक्षो मेरोर्मूर्धनि कृतो मानसोत्तरे कृतेतरभागो यत्र प्रोतं रविरथचक्रं तैलयन्त्र चक्रवद्भ्रमन्मानसोत्तरगिरौ परिभ्रमति ॥१३॥
तस्मिन्नक्षे कृतमूलो द्वितीयोऽक्षस्तुर्यमानेन सम्मितस्तैलयन्त्राक्षवद्ध्रुवे कृतोपरि भागः ॥१४॥
रथनीडस्तु षट्त्रिंशल्लक्षयोजनायतस्तत्तुरीयभागविशालस्तावान्रविरथयुगो यत्र हयाश्छन्दोनामानः सप्तारुणयोजिता वहन्ति देवमादित्यम् ॥१५॥
पुरस्तात्सवितुररुणः पश्चाच्च नियुक्तः सौत्ये कर्मणि किलास्ते ॥१६॥
English translation of Verse 5.21.13 to 5.21.16 which are related to the chariot of Sun:
SB 5.21.13 — The chariot of the sun-god has only one wheel, which is known as Saṁvatsara. The twelve months are calculated to be its twelve spokes, the six seasons are the sections of its rim, and the three cātur-māsya periods are its three-sectioned hub. One side of the axle carrying the wheel rests upon the summit of Mount Sumeru, and the other rests upon Mānasottara Mountain. Affixed to the outer end of the axle, the wheel continuously rotates on Mānasottara Mountain like the wheel of an oil-pressing machine.
SB 5.21.14 — As in an oil-pressing machine, this first axle is attached to a second axle, which is one-fourth as long [3,937,500 yojanas, or 31,500,000 miles]. The upper end of this second axle is attached to Dhruvaloka by a rope of wind.
SB 5.21.15 — My dear King, the carriage of the sun-god’s chariot is estimated to be 3,600,000 yojanas [28,800,000 miles] long and one-fourth as wide [900,000 yojanas, or 7,200,000 miles]. The chariot’s horses, which are named after Gāyatrī and other Vedic meters, are harnessed by Aruṇadeva to a yoke that is also 900,000 yojanas wide. This chariot continuously carries the sun-god.
SB 5.21.16 — Although Aruṇadeva sits in front of the sun-god and is engaged in driving the chariot and controlling the horses, he looks backward toward the sun-god.
You can refer srimadbhagavatam.org for alternative English translation:
(13) This vehicle has only one wheel with twelve spokes [the months], six segments [the seasons] and three pieces to its hub [four month periods], which in its entirety is known as a solar year [a samvatsara]. Its axle is fixed on the top of Meru with Mânasottara at the other end. The wheel of the chariot of the sun being fixed there rotates to the mountain range of Mânasottara like a wheel of an oil press machine.
(14) Fixed to the base of that axle there is a second one which, like with the axle of an oil press machine, measures a quarter of its length. Its upper portion is fixed to Dhruvaloka [the center of the stars].
(15) The inside of the vehicle measures 3.6 million yojanas long and a quarter of that distance wide, it is pulled by seven horses named after the Vedic meters [Gâyatrî, Brihati, Ushnik, Jagatî, Trishthup, Anushthup and Pankti] that, in order to carry the god of the sun, by Arunadeva are harnessed to a yoke as wide as the vehicle [the actual diameter of the sun itself is 1.392 million kilometers].
(16) Even though Aruna, fulfilling his duties as the charioteer, sits in front of the sun god, he looks backward [not to show disrespect].
From Rigveda 1.164:
अस्य वामस्य पलितस्य होतुस्तस्य भ्राता मध्यमो अस्त्यश्नः। तृतीयो भ्राता घृतपृष्ठो अस्यात्रापश्यं विश्पतिं सप्तपुत्रम्॥ १.१६४.०१
सप्त युञ्जन्ति रथमेकचक्रमेको अश्वो वहति सप्तनामा। त्रिनाभि चक्रमजरमनर्वं यत्रेमा विश्वा भुवनाधि तस्थुः॥ १.१६४.०२
इमं रथमधि ये सप्त तस्थुः सप्तचक्रं सप्त वहन्त्यश्वाः। सप्त स्वसारो अभि सं नवन्ते यत्र गवां निहिता सप्त नाम॥ १.१६४.०३
English Translation sacred-texts:
1.OF this benignant Priest, with eld grey-coloured, the brother midmost of the three is lightning. The third is he whose back with oil is sprinkled. Here I behold the Chief with seven male children.
2 Seven to the one-wheeled chariot yoke the Courser; bearing seven names the single Courser draws it. Three-naved the wheel is, sound and undecaying, whereon are resting all these worlds of being.
3 The seven who on the seven-wheeled car are mounted have horses, seven in tale, who draw them onward. Seven Sisters utter songs of praise together, in whom the names of the seven Cows are treasured.
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