## 1 Introduction  
Sunshine duration (SD) is considered to be an important meteorological element since it indicates the amount of solar radiation arriving on the earth's surface and, at the same time, it is also important for various sectors such as tourism, public health, agriculture, energy and climate classification [1; 2; 3; 4]. It is estimated as the total time length in which the direct solar irradiance exceeds the value of 120 W/m\({}^{2}\)[5]. It is measured in units of 'hours per day' or as a'relative sunshine duration' which is the percentage of the astronomical daylength. The available devices to measure sunshine duration can be classified into two types, i.e.,'manual' or 'automatic'. A review of the sunshine recorders, their evolution and operation principles have been carried out by [PERSON] et al. [6]. The traditional instrument for the manual sunshine duration measurements is the Campbell-Stokes sunshine recorder (CS). It measures the bright sunlight as estimated by the length of the burn mark on a piece of specially treated cardboard. It has been routinely measured for over 150 years [7]. [PERSON] [8] found that the irradiance threshold required producing a burn on a sun card varied from 106 to 285 W/m\({}^{2}\), with an average of 170 W/m\({}^{2}\). The heliographic records showed many problems which affect the temporal and spatial analysis of long-term SD series. The most important problems are the gaps in the measurements as well as the change in the location of the station and lack of metadata information. Furthermore, the magnitude of recording of SD depends on the type and quality of the paper, the weather conditions, the properties of the glass in the heliographic balls, as well as the ability of the observers to interpret the card correctly [9]. In contrast, the automatic sunshine duration sensors use a threshold of 120 W/m\({}^{2}\), which is recommended by WMO and are independent of the observers. Therefore, automatic devices have been replacing the CS recorders. Presently, different types of automatic sensors are used, such as Kipp & Zonen CSD1 [10], CSD2 [11] and CSD3 [12; 13; 14; 15], the pyranometric and pyrheliometric methods [16; 17], the Haenni Solar 111 [11], the SONIe automatic device used by the Germany meteorological network [18], the Vaisala DSU12 [19], the Jordan photographic sunshine recorder and the DFC2 photoelectric sunshine meter which is used by China Meteorological Administration [17]. Details about the history of replacing the CS recorders with automatic sensors in various countries can be found in [PERSON] et al. [11], [PERSON] [12], [PERSON] [20], [PERSON] et al. [21] and [PERSON] and [PERSON] [22]. One of the advantages of automatic measurements is that they do not require the daily involvement of personnel to replace the card every day and to read it out. However, the replacement of CS recorders with automatic devices created other problems related to the data quality, comparability and homogeneity of the data series. Therefore, the comparison of'manual' and 'automatic' sensors is required for the interpretation of SD time series. Given the need to compare the'manual' and 'automatic' sensors, various authors have studied the differences in these instruments. For example, [PERSON] et al. [13] compared CS and Kipp & Zonen sunshine duration records from four locations in New Zealand. Generally, the Kipp & Zonen sunshine duration records were higher than CS, with differences ranging from 2% to 17%. Similar results were obtained by [PERSON] and [PERSON] [22] at a station in northern Poland. The higher CSD3 daily SD values compared to CS were attributed to the higher sensitivity threshold required by the CS devices. Furthermore, they found that the higher daily totals obtained by CS rather than by the CSD3 devices are mainly recorded in the summer, due to the so-called 'overburning effect'.