Analysis of information sources in references of the Wikipedia article "Relative hour" in English language version.
At sunrise, the principal part of the day begins, and [as a first resort] it is one's duty to pray when the sun comes out, as it says: 'They shall fear you while the sun endures' (Psalm 72:5). [...] Even though the break of dawn (ʻamūd ha-shaḥar) is considered day for all things, just as we say in chapter 2 of Megillah [20a], as a first resort, the principal part of day is reckoned from the moment the rays of the sun appear and the majority of people have risen [from their sleep] at that hour, for we call [that time] 'and when you rise up' (Deut. 6:7)
The opinion of our Rabbi, the Vilna Gaon, as well as of many other poskim, is that in all the halachic matters touching on the hours [of the day], we reckon [the day] from sunrise until sunset, such as the latest time [one is required to recite] Kiryat Shema, [being] a quarter of the day (a third of the hours), and the latest time of the [morning] prayer, [being] a third of the day (four hours), and the latest time [permitted in] eating leaven on the eve of Passover, [being] a third of the day, and the burning of leaven [which follows] in another relative hour – all [hours being reckoned] from sunrise until sunset. ...The practice is to be stringent in accordance with the Vilna Gaon's method when it comes to the latest time [in which a person recites] Kiryat Shema, and [says] the [morning] prayer, and when it comes to eating and burning the leaven [on the eve of Passover].
{{cite book}}: CS1 maint: location missing publisher (link)The opinion of our Rabbi, the Vilna Gaon, as well as of many other poskim, is that in all the halachic matters touching on the hours [of the day], we reckon [the day] from sunset until sunset, such as the latest time [one is required to recite] Kiryat Shema, [being] a quarter of the day (a third of the hours), and the latest time of the [morning] prayer, [being] a third of the day (four hours), and the latest time [permitted in] eating leaven on the eve of Passover, [being] a third of the day, and the burning of leaven in another relative hour – all [hours being reckoned] from sunrise until sunset. ...The practice is to be stringent in accordance with the Vilna Gaon's method when it comes to the latest time [in which a person recites] Kiryat Shema, and [says] the [morning] prayer, and when it comes to eating and burning the leaven [on the eve of Passover].
{{cite book}}: CS1 maint: location missing publisher (link)Be apprised that all of the hours that are mentioned throughout all the Mishnah are none other than relative hours, and the word relative has the connotation of those hours wherein there are twelve in the daytime, as also at night.
At sunrise, the principal part of the day begins, and [as a first resort] it is one's duty to pray when the sun comes out, as it says: 'They shall fear you while the sun endures' (Psalm 72:5). [...] Even though the break of dawn (ʻamūd ha-shaḥar) is considered day for all things, just as we say in chapter 2 of Megillah [20a], as a first resort, the principal part of day is reckoned from the moment the rays of the sun appear and the majority of people have risen [from their sleep] at that hour, for we call [that time] 'and when you rise up' (Deut. 6:7)
We have reckoned the day from the rise of dawn, since it says in [the Book of] Ezra 'from the break of dawn until the stars came out' (Nehemiah 4:15 [21]). Now he called this [time] 'day', when he said of it, 'in the night they were unto us a guard, while the day [was spent in] labour'
The opinion of our Rabbi, the Vilna Gaon, as well as of many other poskim, is that in all the halachic matters touching on the hours [of the day], we reckon [the day] from sunrise until sunset, such as the latest time [one is required to recite] Kiryat Shema, [being] a quarter of the day (a third of the hours), and the latest time of the [morning] prayer, [being] a third of the day (four hours), and the latest time [permitted in] eating leaven on the eve of Passover, [being] a third of the day, and the burning of leaven [which follows] in another relative hour – all [hours being reckoned] from sunrise until sunset. ...The practice is to be stringent in accordance with the Vilna Gaon's method when it comes to the latest time [in which a person recites] Kiryat Shema, and [says] the [morning] prayer, and when it comes to eating and burning the leaven [on the eve of Passover].
{{cite book}}: CS1 maint: location missing publisher (link)It is known that bayn ha-sh'meshot [= twilight] is a case of doubt whether it is day or night, and they treat it with stringency in every place, the estimation of which time being approximately one-third of an hour after sunset (18 minutes), being from the moment the sun has completely gone down [over the horizon] until the appearance of three stars, just as it has been explained in chapter 5 of Hilkot Shabbat (sect. 4).
{{cite book}}: CS1 maint: location missing publisher (link), s.v. part IV, responsum # 1353 (reprinted in Israel, n.d.)The opinion of our Rabbi, the Vilna Gaon, as well as of many other poskim, is that in all the halachic matters touching on the hours [of the day], we reckon [the day] from sunset until sunset, such as the latest time [one is required to recite] Kiryat Shema, [being] a quarter of the day (a third of the hours), and the latest time of the [morning] prayer, [being] a third of the day (four hours), and the latest time [permitted in] eating leaven on the eve of Passover, [being] a third of the day, and the burning of leaven in another relative hour – all [hours being reckoned] from sunrise until sunset. ...The practice is to be stringent in accordance with the Vilna Gaon's method when it comes to the latest time [in which a person recites] Kiryat Shema, and [says] the [morning] prayer, and when it comes to eating and burning the leaven [on the eve of Passover].
{{cite book}}: CS1 maint: location missing publisher (link)